Netpbm is a toolkit for manipulation of graphic images, including
conversion of images between a variety of different formats.  There
are over 300 separate tools in the package including converters for
about 100 graphics formats.

Examples of supported image manipulations: Shrinking an image by 10%;
Cutting the top half off of an image; Making a mirror image; Creating
a sequence of images that fade from one image to another.

WWW: http://netpbm.sourceforge.net/