http://www.cica.indiana.edu/graphics/image.formats.html has a lot of image format specs, including png and jpeg.

Actually, http://www.dcs.ed.ac.uk/home/mxr/gfx/2d-hi.html is even better, so use that instead.

Oh, and even though it's not particularly graceful, you can sometimes fake an image merge in html. Just make sure you don't have any whitespace between the <img> elements.