POLICY ADDITION
Regarding signatures. While I personally detest images in signatures, I realize that some people find it a necessity, so I will make this allowance for you. If you want to put an image in your signature, you may, but the MAXIMUM size it can be is 468x60. How big is 468x60? It's the same size as the advertising banners you see at the top and bottom of the pages.
Thank you.
While I agree with you and also detest LARGE images in signatures it's not the dimensional size of the image that is the real problem, although it can cause un-necessary scrolling. The real problem is that non-graphics people do not know how to reduce the file size of their image without losing too much quality.
If you look at my image it's 302X81 but it's only 12kb which means it won't take a long time to load even for those with a slow Internet connection. I think you need to have limitations but they should include, dimensional and file size as well as file format i.e. gif, jpg, png