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.
http://www.gimp.org/
Here's the manual
http://docs.gimp.org/en/
Here's some tutorials
http://www.gimp.org/tutorials/
The other much simpler and more basic program that I recommend is located at
irfanview.com
Hope that helps,
Marko