Also, image format conversion while maintaining quality is not as simple as it looks. The answer is, it depends on your audience. If you are fairly sure they can all read foo format that is fine. So it is a case of authoring what the maximum number of people can already accept inline. GIF87a is a safe bet.
Chimera supports just about any format inline, as it is user extendable.
Clearly, this list will evolve over time. This is why the HTML standard does not dictate a list of acceptable inline image formats.