Image weight and dimensions aside (although 480x100 sounds sensible), I think the main problem is that some people never adhered to the COC terms concerning banners in the past, and a certain few were allowed to get away with it.
I don't see this being any different this time around, and so I would suggest a piece of code that checks the dimensions of the banner when posting, and either displays a graphic stating the infringement, or resizes it the maximum allowed.
And there's no reason that this couldn't be done for screenshots either.
Smiley.
If we were allowed to use HTML one could go:
<IMG src="Image URL" width="##" height="##" />
but we aren't
peter