I have heard some positive things about GraphicsMagick when compared to ImageMagick. It seems to have better performance than ImageMagick and potentially fewer resources. ImageMagick has a habit of choking down at times on my vps for certain actions. I believe flickr moved to using GraphicsMagic for their image processing a while back, so that speaks well for it. I have not used it personally, but perhaps someone here can vouch for its improvements over ImageMagic?