On his blog, Vinu Thomas has posted about an "image processing solution" that PHP can use called Asido.

Asido is a PHP image processing solution, with “pluggable” drivers(adapters) for most environments. Asido is still in starting stages with the following drivers supported, with more on the way: GD, ImageMagick, MagickWand.

The library can do all of the typical manipulation bits including resizing, adding watermarks, cropping and converting to other file formats. Check out the project's homepage for more information or just head straight to the downloads and get the latest version.