Excellent set of ImageMagick hacks and howtos.
ImageMagick is an image manipulation software suite for a wide variety of formats, released under a General Public License compatible free license.
From the site, "This web pages presents a set of examples using ImageMagick from the command line. However they are also examples of what can be done using the ImageMagick Application Programming Interface (API). As such these pages should be the first stop for IM users after reading the terse user manuals.
Often the same questions of "How do I..." gets asked, over and over again on the network. The examples in these web pages I hope will answer most of the common 'how-to' questions that arise.
Read how-tos and tutorials at Examples of ImageMagick Usage webpage.