Your favorite image processing package

I'm in the process of creating an image uploader that should be more or less like the WordPress image uploader. Beyond the front end, it will be necessary to have each uploaded image processed, and there would be a thumbnail size, medium, large, and original. I'll want to be able to customize these sizes as much as possible. I haven't got to that point yet, and I know I can do this with CodeIgniter, but I'd prefer to just use existing code if it's out there. No reinventing the wheel. I'll obviously want to have this work with CodeIgniter, but it doesn't necessarily have to be PHP, as long as it can run on any basic hosting environment (Linux).

Do you have a favorite package that accomplishes the image processing? Link me up ...