Unfortunately JPG does not support animation, and progressive encoding is not supported on some platforms (mac) with new browsers or many older browsers. JPG is the best bet for most website images, and also can include embedded comments, copyright and authoring information, which are unfortunately only readable in the programs that created them and can be lost through saves.

The Portable Network Graphics format is a new system similar to JPG, but which uses an unalterable embedded information section to make the images more traceable and supposedly less susceptible to copying. The wisdom of these plans is yet to be proven, and PNG so far is rather poorly supported around the industry. Given time it will probably become more widely used over time as it boasts superior image quality than JPG.