I have yet to find a simple and cheap piece of image editing software for the Mac that doesn't suck. And that includes Seashore.
Seashore/GIMP doesn't suck. At minimum, it makes for very happy customers who previously thought that they had to spend $200 to crop or resize a silly image. We have at least 5-6 customers who are amazed at the "free" Seashore.
Of course, given the very limited amount of image editing I do, "cheap" pretty much means "free". If you're okay with Pixelmator's cost, how about
Acorn?
It is all a sliding scale. I jokingly recommend "The GIMP! It is just as hard to use as Photoshop!!" Me? I get everything I need from Irfanview.
Jim