iOS 11 saves photos in the new HEIC/HEIF format by default on the iPhone 7 or later, and the iPad Pro. This format saves a lot of space; photos are about half the size of JPEGs.
Some people are very worried about this; they want to change back to JPEG. You can do this, but you probably don’t need to. (Unless you’re sticking with macOS Sierra; in which case they look like crap.) Update: some photos don’t look good with High Sierra either, so this isn’t just a problem with the operating system.
To make the change, go to Settings > Camera > Format. Choose Most Compatible, if you want to switch back to JPEG. You’ll see a note explaining that you won’t be able to shoot video at the highest frame rate.
iOS 11 is set up so when you transfer files via email, Messages, AirDrop, or any other type of sharing, these photos are converted to JPEG for compatibility.
Some people have had issues importing these files to certain apps. This could be because they diddled with another setting, that’s in Settings > Photos > Transfer to Mac or PC. This setting should remain on Automatic, unless you do want to transfer the original HEIC/HEIF files.
So don’t sweat it; keep on shooting in the new format, which saves a lot of space. Don’t worry about the files being compatible. And if you do happen to come across .heic files, get a copy of the iMazing HEIC Converter, available for Mac or Windows for free.