If you are looking for a simple, yet efficient and cheap alternative to Photoshop, have a look at Pixelmator.
It’s not free software, but try the demo in order to see what the software is capable of. Pixelmator is a pleasure to use and unlike Gimp, it has perfect integration with iPhoto and effects based upon Coreimage are slick and fast. If only it had more filters (like Gimp’s drop-shadow), it would be perfect.
Here a Pixelmator tutorial you can watch in order to learn how to use the beast.