At some point, you want to get started doing photo editing, composition, or even original digital art - definitely more than you can do in Picasa or iPhoto - but you don't want to shell out $600 or more on a current or recent version of Photoshop (the acknowledged standard for professionals). Fortunately there are options. Pixelmator and Acorn.
Both of these programs are fast, effective, powerful enough for anything short of a "pro" workflow, and most of all, because they don't have all of the pro features, cheap. Both also use automator scripting to do repetitive or batch processing.
Pixelmator looks and feels more like photoshop, with floating palettes for tools, color selection, etc.. Acorn tries to get it all done in one tools window with options that change based on the current tool in use. Acorn also lets you do some very basic editing with a restricted feature set after the trial is over.
The choice is more a matter of personal style, but both are solid products I can highly recommend.
I've got to note that, with the introduction of the Mac App Store, Pixelmator is now available for an incredible price: $30. When you factor in that this is effectively a family/household license, the deal gets even sweeter. And while no, it's not Photoshop, it is - as this Ars Technica review notes, an absolutely killer program, and one heck of a deal.