virtually all of the tasks will involve manipulating .gif and .jpeg images. Chores will likely be limited to creating thumbnails, cropping, rotating, and weight reduction
That all sounds like imagemagick to me. It'll even still handle gifs, iirc. And you will probably run into the same limitation with GD as i did: useless for photographic images because it can't handle anything over 8bit colour. But that was due to change in version 2.ish, so perhaps I'm out of date.
Generally I'd agree with others here: GD for image creation and IM for processing images that already exist. I'm off to have a look at that Imager, though :)