You can save an image in PNG, JPEG, GIF and dozen other formats. I have a detailed guide on choosing file formats for your images but if you still feeling confused, this slightly exaggerated comic courtesy Louis should help you pick the right file format.
Which File Format Should You Choose?
The help manual of a popular screen capture program offers the following suggestions
GIF format is limited to 256 colors and is a lossless compression file format, a common choice for use on the Web. GIF is a good choice for storing line drawings, text, and iconic graphics at a small file size.
PNG format is a lossless compression file format, which makes it a common choice for use on the Web. PNG is a good choice for storing line drawings, text, and iconic graphics at a small file size.
JPG format is a lossy compressed file format. This makes it useful for storing photographs at a smaller size than a BMP. JPG is a common choice for use on the Web because it is compressed. For storing line drawings, text, and iconic graphics at a smaller file size, GIF or PNG are better choices because they are lossless.
George adds – “JPEGs are for photographs and realistic images. PNGs are for line art, text-heavy images, and images with few colors. GIFs are just fail.”
Randall Munroe at xkcd and Matthew Inman at Oatmeal mostly use PNG for their web comics while Hugh MacLeod at GapingVoid uses the JPEG file format for his cartoons. Also note that the image illustration above is in PNG format.