GIMP (GNU Image Manipulation Program) is a free program that you can use for photo retouching, image composition and image authoring among other things.
GIMP can be used on different platforms (Windows, Linux, Mac) and for a wide variety of tasks. It can be extended with additional plug-ins.
GIMP has an active user community and support material; instructions and tutorials can be found in abundance online.
VISIT THE GIMP WEBSITE
VISIT GIMP TUTORIALS