I'd suggest that signature images are limited to 468 x 60 for width and height and 50k as a file size. This does not mean you can have any variation of an image that adds up to 28,080 total pixel size. The 468 x 60 size is allowed because it's short enough to not mess up pages vertically, and wide enough to put plenty of info/pics. 468 x 60 is a standard internet banner size and companies use it to advertise their businesses, so it's certainly big enough to contain anything wanted in a signature image.
ps. Exp, may want to add that to the rules.
