what I did was I got rid of html and the img code, then I made a new bbcode called [sig], and put this as the html it relpaced.
Code:
<img src="{param}" height="101" width="455">
This will make every image have the dimensions 455x101, but people will soon realize they need to make new sig images that will look good in those dimensions. If you would like to see what that looks like, then just look at my forums in my sig.