Another idea from me... Is there a way to have a feature when editing forum profile information to have a box that allows people to put a link to a picture in, with options of specifying it's justification (left, centre or right) instead of using [IMG] code in the text?
That was you could force it to a limit of 150 pixels or whatever to help reduce the number of oversized signature images that are creeping in.
Also, ban images covered in brand logos. It's free advertising which isn't allowed.