Nice hack Dorign. Simple and edits very easy (clicks' install).
Here's an example on my forum (3.0.9):
http://www.shipmodeling.net/vb_forum/thread907.html
Just what I needed. The original image my user posted was over 1100 px's wide. I set my limit to no more than 600 px's.
Tested it with IE, Firefox and Nutscape. All seem ok
PS, I added a little notice to my users in the "Post Thread' and in the 'Edit Post' templates to let them know that any over sized image they link to will be resized automatically so they don't think they're going crazy.