Manipulating Images

"In the process of preparing an image for the Web it is frequently necessary to manipulate it in some way, for example to change its size, color palette, or other attributes. The tools listed here are for use on Unix systems; there are analogous tools for other platforms.

• Displaying an Image
• Getting Information About an Image
• Converting Between Formats
• Cropping
• Scaling
• Interlacing
• Transparency
• Painting / Touching Up"

http://www.geom.uiuc.edu/events/courses/1996/cmwh/Stills/manipulating.html