Pixelmator is a super-slick image editor that is perfect for every little change you need to make.
It loads super fast and is so pretty. The way it handles effects is a little too pretty actually.
Migration Score
BAD
Migration Options
The GIMP
Pixelmator can export to Photoshop format which GIMP can then read. Do some checking with it though- it seems like some of them work well while others don't. May be the color depth.