How about limiting the size of an image to use in signatures to like 10k and maybe 200 X 40 pixels or something like that? That would fit nicely, and make it so it would have to be a remote linked image so as to not waste your bandwidth.
Besides that, you could write a script that would disable images in signatures only that could be set in the user CP, by default img's in signatures would be set to 'off' in their CP's.
It would be awesome. great idea! let's do it!