"gd is an open-source library that allows users to create and manipulate images easily. It lets you open images in formats such as JPEG, PNG, XPM and a few more. gd works something like this: it opens images in different formats and converts them to generic bit-mapped images in memory. It then lets you do graphical operations, such as drawing lines, arcs, ellipses or rectangles on that image, and stores the resulting image in any of the earlier-mentioned formats. For example, you could write a simple command-line program that converts a given file in JPEG format to PNG using gd. gd also can change colors in the image and copy, cut, merge or rotate it..."
