Bandwidth isn't the only reason there's a sig limit. Loading images takes RAM. I can crash a browser by displaying enough of the same image a million times, especially animated ones. (At least I used to be able to with IE.) It's also visually annoying for people who have sigs enabled, even if a browser is capable of loading it.

It's also a rule, and the rules here are arbitrary based upon how we want to run the place. If we decided all sigs can only be green, then sigs that aren't green would be removed. Come to think of it... nah.