Ok, it's not exactly what you are looking for but you can try it.
Get the signature hack and install it (http://vbulletin.com/forum/showthrea...threadid=12282).

Then edit the template where users can change their signature so that they cannot edit it directly (delete the textarea) but have to load the signature editor.

In the signature editor php file (sedit.php) you can make all changes you like.

Currently it's set to a limit of 4 lines and the preset number of images but it can't be that hard to define a number of images and change the limit to a number of characters and add image size restriction.

It's not an easy way but a possibility.