Photoscape is a free tool used by a number of the CG administrators. It provides a lot of the functionality similar to Adobe Photoshop (which is the gold standard insofar as photo editing). For day-to-day type photo editing, mainly the types of things you would need for chips and such, it is a very reasonable easy tool to use. If you needed to cut people out of pictures, change backgrounds, remove powerlines, etc. then you need something heavier and it won't be free.