The top discussion explains how to change file sizes using photoshop.

Below that is a description of the difference between jpg and png files.


Changing File Size

Changing file sizes can be a source of confusion for many. The examples below are from the latest version of Photoshop CC.  Your software and operating system may display differently, but the principles will be the same.


There are two methods for changing files sizes

1)  Change print width or height in inches, plus resolution.
2) Change pixel width or height only.

You never have to use both.  Each does the same job using different information.


Method 1 - print size plus resolution

Note: in the image size dialogue above that the resample box is checked.  If you leave it checked and make a change, it will make a permanent change to the actual file size. To avoid that, uncheck the resample box.  See what happens below?

Notice that when you uncheck the resample box the pixels dialogue is replaced by inches.  Use this method to simply change the printing size without changing the file size.  Note that if the resolution becomes less than 240 pixels/inch, the quality will not be good.  In this case you can upsize the image.  First check to see what the  print size would be at 240 pixels/inch. See Below:

To check that, I typed 240 into the resolution box and notice that the image size in width changed to 6.25 wide by 9.025 high.  So it isn't recommended to print any larger than this without upscaling the image.  You can usually upscale the image by about twice the long dimension if it is sharp to begin with and get good quality.  This can vary. See Below:

So to upscale the image to print that is 6.925x10, I first checked the resample box and changed the height to 10 and entered a resolution of 300 pixels/inch.  Note that I used Preserve Details 2.0.  I find this works best for upsizing.  It may not be available in your image processor, so experiment with the dropdown choices.  With Photoshop, you can click on the image at the left of this box and see a before and after upsize result.

A simple guide is if you use 300 pixels/inch (or dpi) then multiply the length in inches by 300. (300x10=3000).  This will yield a 10 inch high print.

For a 7 inch high print use (7x300=2100) so that would be for a 7" high image in pixel dimensions.


Method 2 - Pixels/inch only

Here's a practical example:  
A gallery asks you to send an image that is 1200 pixels at the long dimension.  What to do?

First be sure the Resample box is checked and Pixels are selected for the longest dimension.  Then type in 1200.  That's all you have to do.  Ignore the resolution setting.  Below is a further exploration of the concept.

All I did was change the inches dropdown to pixels and notice that the height is now 3000 pixels.  With this method you don't need to worry about the resolution box at all.  Any image that has pixel height at 3000 will make a good print at 10 inches high regardless of the resolution setting. That being said, when you actually go to print it, you will have to revert to the Type 1 method and leaving the Resample Box unchecked, change the Resolution to the best match for your printer.  If it happens to be 300 Pixels/in then the image size will automatically go to 10 inches high without having to resize the image.  Note: the Resolution doesn't have to match your printer's resolution.  Most printers will print the size designated using an internal software conversion method.  I can tell no difference unless the resolution is below 240.  Most people cannot detect image quality improvements above 300 PPI (DPI).  That's not to say that printers of the future might improve enough to make those changes noticable.  Perhaps it's the human eye that cannot detect higher resolution.  My Epson actually converts the dot pattern to 2880 dpi to spray the ink on the paper.  If you use a magnifier to view an inkjet print you will see the ink spray pattern on the paper.  This is sort of like looking at a half screen photo in newsprint, only that's much more pronounced depending on the printing press methods used.

So if you notice I changed the resolution to 72 and the pixel height to 3000.  this is exactly the same as the previous file size.  

Note: the resolution stayed the same and actually this will give you a 7" high print  (300pixels/inch X 7 = 2100)

So if a gallery asks for a 5x7 at 300dpi, just make the image about 2100 pixels at the long dimension and ignore the resolution setting unless they specifically ask you to change that.  The result will be close enough for their needs.

Hope this helps!


JPG VS PNG

A question has come up.  Which to use jpg or png?  The short answer is both work well for website or email use.  For printing however I recommend either tiff or Psd files.   Both jpg and png files are compressed files, thought the png file is supposed to be lossless, meaning resaving doesn't result in data loss.  Also the png can be a transparent file.

EXIF dat is not saved, so shutter, speed, f/stop and ISO data won't accompany the file.

Feel free to read the data below or search other sites.

Copied from the Digital Trends website:

JPEG

Short for Joint Photographic Experts Group — the team that developed the format — JPEG has become the standard compressed format in digital photography and online image sharing due to its careful balance of file size and image quality.

The exact ratio differs depending on the program and settings used, but the typical JPEG image has a 10:1 compression ratio. That is, if you start with a 10MB image and export it as a JPEG, you should end up with an image that’s roughly 1MB. A JPEG should have almost zero perceptible difference in quality, although this depends on the content and file type of the original image.

To do this, JPEG relies on discrete cosine transform (DCT). While the math behind it is complicated, this compression algorithm takes a look at the entire image, determines which pixels in the image are similar enough to the ones around it, and merges the pixels together in tiles (groups of pixels that have the same value).

This method is extremely efficient but comes at the cost of throwing away information you can’t get back. JPEG images (with a few exceptions mentioned below) are lossy, which means after the image is saved, the data that was lost can’t be recovered. So, just like making a photocopy of a photocopy, each time you open and save a JPEG, it will look slightly worse than before.

For this reason, JPEG is not suggested as an archival image format, because if you ever need to open it and make edits again, you incur a loss of quality. Nondestructive photo editors, like Adobe Lightroom, can help skirt around this issue provided you never delete your original files, as they only save edits as metadata rather than writing over the original image.

JPEG should also be avoided with text-heavy images or illustrations with sharp lines, as defined lines tend to get blurred due to anti-aliasing. (Anti-aliasing is an intentional blurring designed to eliminate rough edges.) As you can see in the image below, a screenshot taken from our homepage, the text and white background show a lot of artifacts on the JPEG (right) compared to the PNG (left).

That said, there are times when you need to turn formats like PDFs into JPEGs. In those instances, it is best to ensure you export it at the highest quality settings to ensure all of the text is sharp.

JPEG supports both RGB and CMYK color spaces in 24-bit, but its CMYK offerings leave much to be desired. (Modern printers handle RGB files just fine, so this isn’t a huge issue. You should still stick with higher-quality formats for printing, however.) An 8-bit grayscale is also an option but the compression ratios are far less impressive with grayscale compared to color images.

Over the years, many variations of JPEG have come and gone. For example, JPG-LS was designed to fix the issue of lossy compression, but it never gained a foothold and eventually fell to the wayside. JPG2000 also attempted to address the lossless issue, but it, too, failed to gain traction. BPG, a new format based on the H.265 video standard, was determined to take over JPEG, but never really caught on.

The creators of the JPEG recently shared a new format designed not to replace the JPEG but to exist alongside it as an option for faster streaming. In a JPEG XS, the compression is only six times instead of 10, but simpler algorithms mean the file is faster for tasks like streaming. A potential replacement could come in the form of HEIF, which is also based on the h.265 standard. Where others have failed, HEIF could succeed thanks to the support of one of the biggest brands in tech: Apple. It still has a ways to go, but may only be a matter of time before it is as widespread as JPEG is today.

PNG

An acronym for Portable Network Graphics, PNG is a lossless file format designed as a more open alternative to Graphics Interchange Format (GIF).

Unlike JPEG, which relies on DCT compression, PNG uses LZW compression — the same as used by GIF and TIFF formats. Boiled down, PNG’s two-stage LZW compression takes strings of bits contained in the image’s data, then matches those longer sequences to accompanying shortcodes held in a dictionary (sometimes referred to as a codebook) that is stored within the image file. The result is a smaller file that maintains high quality.

The biggest advantage of PNG over JPEG is that the compression is lossless, meaning there is no loss in quality each time it is opened and saved again. 

PNG also handles detailed, high-contrast images well. It’s for this reason PNG is more often than not the default file format for screenshots, as it can provide an almost perfect pixel-for-pixel representation of the screen, rather than compressing groups of pixels together.

One of the standout features of PNG is its support of transparency. With both color and grayscale images, pixels in PNG files can be transparent. This allows you to create images that neatly overlay with the content of an image or website. As seen in the GIF above, many editing programs — in this case Adobe Photoshop Mix — use a checkered background to indicate the transparency of a graphic. This makes PNG great for logos, particularly those with text, used on a website. If you create a transparent background in Photoshop and save the images a JPG, on the other hand, that transparent background just becomes white because the format doesn’t support transparency.

When it comes to photography, PNG might seem like a solid alternative to proprietary RAW formats for lossless image storage, but the truth is there are plenty of better alternatives, such as Adobe’s Digital Negative (DNG) — which you can even shoot on your smartphone — and TIFF. 

PNG also doesn’t natively support EXIF data, which includes information such as shutter speed, aperture, and ISO from the camera it was captured with.

PNG was made for the web and it has proven its worth. JPEG might be the format of a majority of the images, but PNG occupies an important niche that JPEG can’t effectively reach, and is basically the only choice when you need to clearly render a logo or text over other elements on a website.

Much like JPEG, PNG has also had a few variations throughout the years. APNG is a still-supported format designed to replicate the animated functionality of GIFs. It’s not nearly as prevalent but is supported by many modern browsers.


Another fun tidbit is that in the early stages of PNG’s development, it was suggested it be called PING, an acronym for “PING Is Not GIF,” a cheeky dig at the creators of the GIF format

Using Format