ImageMagick

• ImageMagick   - A collection of tools and libraries to read, write, and manipulate an image in any of the more popular image formats. (Unix/Linux/Win 95/98/2000/Mac/VMS/OS/2)

• Freshmeat.net: Project details for ImageMagick - Project description, downloads, article links, and user comments.
• Graphics from the Command Line - Article that demonstrates that ImageMagick command line tools can be used to flip, size, rotate, and stylize various file format images in batch.
• ImageMagick Hacks - A list of tools that use ImageMagick and articles that explain its use.