They are simply image files. You can save an image as a .gif file =(low quality, but small file size) or a .Jpeg (Compromise between quality and file size).
If yur on a computer, open the folder (not the file, the folder) which the image is saved in. Right click, go to 'view' and click 'details'. This should arrange all the files in this folder, so you can see which images are of what file type.