Gimp is quite a powerful free program and does many of the same things photoshop does - it is highly recommended. Like any other program that is powerful, there is a learning curve but it's not too huge and there are tutorials and manuals to help.
I am just a rookie with Gimp but if members can't figure something out I can always try to help (Just PM me)....and MANY people use Gimp so you can also Google what you are after.
Here's the manual
Here's some tutorials
The other much simpler and more basic program that I recommend is located at irfanview.com
Hope that helps,