From awcolley.com

Bookmarks: Imaging and Graphics Bookmarks

Imaging and Graphics

Computer Programs

Free (as in requires no money) and works on Windows/Cygwin

The GIMP

• GIMP home page
• GIMP for Windows
• GIMP User Manual
• Grokking the GIMP --- understand image manipulation concepts, know which GIMP tools are most effective, and learn savvy use of these tools
• GIMPguru.org --- a resource for those using the GNU Image Manipulation Program (GIMP) to edit photographs

ImageMagick

• ImageMagick home page
• Command line tools (documentation)
• Examples of ImageMagick Usage (Version 6)

Netpbm

• Netpbm home page
• User's Manual

Others

Commercial

Paint Shop Pro

Photoshop