GIMP is definitely my favorite, next is Paint.Net . If you feel a little lost using them at first, it does take a while to get used to. Since you have a specific task that involves photos, you might want to try the editing tools that come with Picasa2. Although the tools in P2 arent as powerful as GIMP, they are specifically designed for editing/enhancing photos, and are relatively straight-forward. My only misgivings for P2 is the way it handles file/directories, which can be kind of confusing. Also I'm not sure if you can work with glare issues in P2, but it might be worth a shot.
But whichever you go with, good luck with it.