GIMP is a versatile graphics manipulation package.

http://www.gimp.org/downloads/

It  is a freely distributed program for such tasks  as photo retouching, image composition and image authoring.

It has many capabilities. It   can be used as a simple paint program, an expert quality photo retouching   program, an online batch processing system, a mass production image   renderer, an image format converter, etc.

GIMP is expandable and extensible. It is designed to be   augmented with plug-ins and extensions to do just about anything. The   advanced scripting interface allows everything from the simplest task to   the most complex image manipulation procedures to be easily scripted.

25 Gimp video tutorials:
http://sixrevisions.com/graphics-design/gimp_video_tutorials/

There are many tutorials about Gimp on youtube and elsewhere, here is just one of them…