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The short and simple answer is that screen shots are often of web content, and that content often has text. The Portable Network Graphics (PNG) format does better with images like that, and the resulting file size is often correspondingly larger than a Joint Photographic Experts Group (JPG) file. You probably don't want much compression here.
Photographs, on the other hand, tend to be images of surroundings, indoor or outdoor, and generally should be as small as possible because people take a lot more photos with their iPhones than screen shots. JPG is a compressed image format, and is designed to work with the kinds of images that a camera produces.
Classic photographic image uses JPG.
Recently, I ran across several articles on the Internet that explain, in terrific detail, why these two different formats are used for different jobs.
How to Choose an Image Format for Screenshots
What’s the Difference Between JPG, PNG, and GIF?
In both these articles, the authors explain the benefits of each format and why PNG is better for screen shots and JPG is better for photography. In the second article above, the author writes:
- PNG is good option for transparency and non-lossy, smaller files. Larger files, not so much, unless you demand non-lossy images.
- JPG is still the king for photographs and photo-like images on the internet, but be careful, as your file can degrade with every save.
Classic screen shot of a web page using PNG.
Changing the Default Format – NOT!
It's not easy to change the default image format on an iPhone, say, for screen shots, like you can on a Mac because you don't have access to the iOS UNIX shell on a command line. I'm told there are some apps that can change the format, but that can also cause problems when you try to export the photo later.
It's a very good idea to go with Apple's technical choice here. If you need to manipulate the image, use AirDrop or email the image to your Mac or PC where there are eleventy-zillion different apps that can edit the image and change the file format.
And now you know why your iPhone's screen shots use PNG and camera photos use JPG.