ImageMagick - Powerful Programmatic Image Processing
ImageMagick can read, write, and convert between some 100 different image file formats, including all commonly used ones such as GIF, JPEG, PDF, PNG, Postscript, and TIFF. Among the numerous capabilities of ImageMagick are image resizing, rotation, cropping, flipping, trimming, making parts of an image invisible, adding shapes or text to an image, various special effects, creating animated GIFs, combining images, and handling giga-pixel images.
Binary installation files for various operating systems are available here.


Comments
Image Magic is a very handy way to resize and thumbnail images. We used it extensively to process large images. One thing to note is if the code crashes it can leave behind VERY large file in your /tmp directory. Be on the lookout for them.