For future references you might want to book mark this web site that will help you searching for alternatives, named "Alternativeto".
GIMP made it to the top of they list, but there is multitude of alterntives listed.
Open Source by Free Software Fondation (GNU) | Mac, Windows, Linux,
GIMP is a freely distributed program for such tasks as photo
retouching, image composition and image authoring. It can be used as a
simple paint program, an expert quality photo retouching program, an