Hm hm. Personally, I put a size limit if I do my own siggy (signatures; which is on gimp and/or scratch for animation) if it's a image. I'll rid one pic (but it isn't that big anyways).
Maybe a size limit for pictures would be good, though. I don't want the page to break because of an oversized picture as a siggy, no less take up space and make the page longer than it actually is (more scrolling down, which is a pain for people who have mousepads on laptops, causes arthritis if repetively used in long periods of time). Size limit on pictures should be good, which is pretty much reinstating what Red just said.