I want to created automatically rounded corner thumbnails for a particular project I'm working on, using something like this: http://webdeveloperplus.com/php/create-thumbnail-images-with-rounded-corners/

I suppose I could create a function that mods it upon first view, then cache that, but what I'd ideally like to do is find a way to hook something like this into the thumbnail creation process itself.

Any clues on where to start?

EDIT: Not in CSS. I'm using border-radius all over the site, but the images specifically need to be rounded on the server side.