Just a comment (or warning...) about the Gimp.
If you are happy editing jpg images (for example, the digital photos from your compact camera), gimp is just perfect. BUT if you need something more professional, like 16 or 32 bits per colour channel (jpegs have only 8 bits per colour channel) or to work on several different colour spaces, gimp is not for you... yet. 32 bits per colour channel is in the to-do list for the Gimp project, but it is not yet implemented.
Free programs that support up to 32 bits per colour channel are:
CinePaint (an early fork of the Gimp project): it is used on the cinema industry. I think versions for Windows and Mac are available.
Krita: This amazing program is part of the koffice suite. koffice version 2 will be available for Windows and Mac.
For small (and not so small) works, imageJ.
But again, if you only need to edit the last birthday photos, the Gimp is just perfect.
There are two types of people: the ones that believe that there are two types of people and the ones that not.
openSUSE 12.2 with KDE SC 4.8.5 / AOO 3.4.1