The GIMP (GNU Image Manipulation Program) has become the standard Open Source software to create and manipulate raster graphics (photos). It boasts most of the features of the expensive commercial counterparts (such as PhotoShop) without the price tag, and perhaps without the flashy user interface.

Virtually all flavors of linux come with the GIMP preinstalled. If you need to you can find the appropriate version of the GIMP for your distribution from here

A Windows version is available from here

And, for completeness, Mac OS X users should refer to this page

GIMP comes with a built-in help system. Once you have started the program, press F1 for context-sensitive help. You may have to install the help pages from a separate package (gimp-help), depending on how your version of GIMP was packaged. Check here for details.

For tutorials and tips I suggest Googling. But to get a head start you might like to refer to Grokking the GIMP in it’s on-line or printed form.