i've done the following for my board:

-maximum image filesize in sig (single image and combined)
-maximum image resolution (width by height)
-per user hardcoded (ie. no cp stuff) sig disabler

its not foolproof (i can think of about 3 ways to get past either the resolution or filesize thangs, since its only checked on user profile edit) but it works for me for the mostpart. i'm working on removing width and height tags from images as well - i've got some idiots on my board taking small images like smilies and making them enormous in size since they cant just use huge images with varying degrees of luck.

i didnt think anyone else'd use this stuff so i just did it really quick and its reaaaaaaally ugly but if anyone is interested i can clean it up and post it.