G.I.M.P. can take a while to learn to use and can have a pretty steep curve (steeper than photoshop), but this is made up for in the fact that it is free (free as in speech AND free as in beer). It is also pretty much the only mature image manipulation program made for a non-windows OS (Unix/Linux, FreeBSD).
There are litterally thousands of tutorials out there that you can use to teach you G.I.M.P. especially since it is open source software, and is pretty wideley used. As some advice, never assume G.I.M.P. cannot do something, because it can, you just haven't found out how yet.
As a start I would suggest the main tutorial site: http://www.gimp.org/tutorials/