“The best rule of thumb for bloggers using images in their posts is:
Use JPGs for photographs, landscapes, and faces. Use PNGs for charts, graphs, screenshots and cartoons.
Use GIFs only for animations, and sparingly.
As with all rules of thumbs, you should use your best judgment. You should aim to understand how these two image formats work so you can make the right decision.”
See it all at www.hanselman.com.