Solution:
- Set a DEFINED limit of pixels.  "Can't be much larger than" 468x60 isn't a hard limit, thus people will press past it.  Define the limit AS 468x60, and self-policing within the community will happen.  There is no reason not to have a hard limit, and plenty of reasons to have one.
- Ban anyone who does not follow the rules on it.  If people know the consequences are severe, they will be much less likely to break said rules.

Also, the next forum software will disable all image embedding?  Whoa, talk about a step backwards!  I guess some people really DO still want to be stuck in the 90's.