Creating Square Thumbnails of Images

Anyone any pointers on how I might go about hacking my wordpress install so that it automatically creates square thumbnails of a settable size when I insert a picture or gallery into a blog post, and wraps then in link tags pointing to the full-size image?

I assume gd is the way to go, and I have some experience of PHP, but never done any work with WordPress theme/template files before.

Any advice much appreciated. If there are any plugins already out there that do the job, then let me know!

Cheers!

Alex Drinkwater
Media & Communications Dept.
Goldsmiths University of London