Did you take a look at
phpThumb() http://phpthumb.sourceforge.net/ ?
In my opinion is far better than Smart Image Resizer. With a lot of options in the URL like Unsharp Mask, Clip, Quality, etc.
Did anyone use it with X-Cart?
Could you please give me more idea about Thumb Generators in PHP (Image Resizers)? I am trying to find one better the phpThumb.