It supports over 68 image formats and make no mistake this product differs in 2 main ways from others you might find, it is completely free and everything can be automated! The PPWIZARD thumb nail header runs it in command line mode but there is a GUI ('X' windows).
The source is available and it runs on Windows 2000 or Windows 95/98 as well as on virtually any Unix system and Linux, Macintosh, VMS, and OS/2. It is available from http://www.imagemagick.org/
One glitch (at least under windows), it must be installed in the "C:\ImageMagick" directory for it to work.
There is a "linux" patch for this product so that it will run on Linux.