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Full user review
"The Swiss Army knife of command-line image processing."
ImageMagick provides a powerful set of tools for non-interactive image processing on the command line and through its application programming interface.
The included graphical interface is rudimentary at best.
I'm writing this mostly to clear up any misconceptions users might have about this software. ImageMagick is indeed a powerful and robust collection of tools; however, its strength is on the command line, not in its graphical interface. ImageMagick will not replace a program like Photoshop for interactive image manipulation, but it will come in handy for batch-converting between image file formats, automatically generating thumbnails on a website, and other such "behind-the-scenes" tasks. (It's capable of much more, too; take a look at the included documentation for many more examples.)
ImageMagick is not something most people will ever need to use, but if you ever have to process a large number of images in bulk, it is an absolute godsend. If Photoshop is a paintbrush, think of ImageMagick as a printing press; it's not really a tool for careful refinement, but it's by far the more practical choice for mass production.