Gimp

Use – open source image editing program
Cost – Free 

Description:
GIMP (the GNU Image Manipulation Program, also referred to as ‘The GIMP’,) is a free open source image editing program. It has a comprehensive set of tools comparable to those available in commercial software such as Adobe Photoshop.
GIMP can help you with simple and advanced image editing tasks, such as photo retouching, image composition, image optimisation and image creation.

Tutorials:
http://www.tasi.ac.uk/advice/creating/gimp.html

Developer: www.gimp.org