ImageMagick is a set of tools that can be used to read, write, and manipulate images in almost any format including TIFF, JPEG, PNG, PDF, PhotoCD, and GIF.
The advantage of ImageMagick is that it can be used to create images dynamically making it suitable for Web apps. In addition, you can resize, rotate, sharpen, color reduce and add special effects to an image. However, ImageMagick is strictly for those familiar with the command line as there is no GUI. You can only make changes to images using commands such as "$magick> convert rose.jpg rose.png".
For those familiar with programming languages, you can use C, C++, Perl, Java, PHP, Python, or Ruby to make changes. A high-quality 2D renderer is also included which provides SVG capabilities. If you are familiar with programming commands, then the range of actions possible with ImageMagick is truly impressive including, conversions, resizing, transparencies, animated GIFs and shape drawing. However, the lack of GUI will exclude most people from using it.
If you're familiar with programming languages and need a powerful command line based tool, Image Magick is worth exploring.