For photos, Jpeg is a good format to use as it produces small file sizes without too much noticeable loss. However, all Jpeg files are saved with 16.7 million colour depth, so if you have a monochrome image such as text or a technical drawing, PNG (Portable Network Graphic) will produce a clearer image and a smaller file size. PNG is similar to GIF (Graphic interchange format) but superior in some ways.
I've just downloaded a drawing off the internet. The file sizes are as follows:
GIF - 15KB
JPG - 59KB
PNG - 7KP
The JPG one will be almost unreadable and unprintable because it is converted to a 16.7 million greyscale. The PNG and GIF files are much better. I'd post them here if it would let me