Most forums that allow images in signatures have a clearly defined size limit.
I think that this would greatly help to clean up the forum and to make it more readable.
I think I've only seen one person with a signature image larger than the two posters above us. I don't see this as really helping forum readability, personally. But I think a size limit is a good rule to put in place in case people DO start abusing their sigs. I just haven't seen much abuse of it myself.