GIMP is a very nice, and free graphics and photo editor that has quite a bit of power.
But, it has a very quirky interface. And, if you've used other programs like Photoshop or Paint Shop Pro, it can be hard to get to used to.
Here's a possible solution: an application that modifies the GIMP interface so that it looks like Photoshop. Underneath it is GIMP. Up front, it looks pretty close to Photoshop. If you just can't get used to the GIMP interface, this might be a great option for you. Give it a look: www.gimpshop.com.
This thing is close enough to photoshop that you can actually use photoshop tutorials just as they are. The few places where they are different, well, you can figure them out.