The GIMP, at least in its usual form, need not apply. Perhaps I've been corrupted by Photoshop, but regardless, the GIMP just feels unusably arcane and clunky. Nothing is where I expect it to be, and I spend too much time fighting the interface. I'm going to keep using Photoshop on the desktop machine, and I really, really don't want to have to expend a lot of mental effort on switching back and forth. (Update: I tried GIMPshop, which is supposedly reworked to be much closer to Photoshop, but the latest version of that is based on the GIMP version 2.2.11, which isn't compatible with Leopard.)
I've tried several editors, and one, Acorn, almost works for me. It's small, quick, cheap ($39), and works as I expect. Another, Pixelmator, has a user interface that's closer to Photoshop, but with added flash; it's a bit more expensive ($59), though.
There's only one problem: One task I perform regularly is to take an image from a camera, crop it while preserving the aspect ratio, then resize it to 640 pixels on the long dimension, again preserving the aspect ratio. This is simple in Photoshop: you select the Crop tool, use it to select the entire canvas, then hold down option and shift while dragging a corner of the crop frame.
None of the editors I tried (in addition to Acorn and Pixelmator, I also tried Pixel (a very different program), Seashore (a cut-down GIMP with a somewhat reworked user interface), and LiveQuartz) made that operation even possible. Those that allowed altering the crop frame had no way to constrain its proportions; some didn't allow altering the crop frame at all.
Grumble. Any other suggestions out there? I'd rather not spend over $100 if I can help it.