Ironically there was a
review of Acorn just posted this morning which I think might be what you're looking for.
You may also want to check out
Gimp for OS X which will do everything Photoshop does with a slightly steeper learning curve. Gimp is free.
Pictureaque is targetting the 'simple yet powerful' space of image editing and for $20 you get a full copy.
Hope that helps.