Commercial graphic editing software can be very expensive. The list below contains open source or free alternative graphic design software.
Gimp
Gimp is free and can be used in Windows, Mac or Unix environments. A user manual is available.
Paint.NET (PC only)
This software was started as part of a college project and has continued to be developed. Paint.NET can be enhanced with the use of plugins. Paint.NET only works in Windows. A user manual is available.
Pinta (Mac only)
Pinta is like Paint.NET except it is for Macs.
Inkscape
Inkscape is a free open-source vector graphics editor similar to Adobe Illustrator or CorelDRAW. Tutorials and other help options are available.