GIMP (Gnu Image Manipulation Program) is a popular and free open source cross-platform image editor available in versions for Linux, Windows, and Mac. It is seen as a viable alternative to Photoshop.
GIMP provides the tools needed for high quality image manipulation from retouching to restoring to creative composites. It also offers graphic design elements.
This course offers short tutorials that will help you learn how to use GIMP to manipulate and improve images.