GIMP

I know GIMP has a funny name, but it might come in handy for you.

Gimp (short for Gnu Image Manipulation Program) is a free, open-source photo editing program. It has many features that some programs costing $100 don't have, and some features you will only find in a $600 program.

The one major drawback to GIMP is it does not read files saved as CMYK, nor can it write files as CMYK. (Although there are some plug-ins available that are supposed to add some CMYK support.) If you know what CMYK means, then you can see the possible problem. If you don't know what CMYK is, and most people don't, then you probably don't need to worry about this problem.

GIMP runs on Windows, Linux, and Mac OSX.

Check out the GIMP website