One other program to consider is GraphicConverter, which Apple provides as part of the included bundle in each PowerMac G5 (presumably on the Mac Pros too). This is a surprisingly capable program. Well worth checking out and spending some time with.
However, the real royalty in this area is Photoshop, then Photoshop Elements and then GIMP, in that order.
Note that GIMP does not understand Color Management at all. Photoshop has full Color Management support and Photoshop Elements is somewhere in between, with partial support for color spaces. This continues to make Photoshop itself the king for real graphic professionsals.