Here's a website listing 12 free alternatives to Photoshop:
http://beebom.com/best-free-photoshop-alternatives/I tried Gimp once and it drove me crazy, because I was used to Photoshop and Gimp is significantly different (or at least it was at that time). But it seems to be the program that everybody recommends as the best alternative to Photoshop. If you're willing to work through the learning curve, it will probably serve you well.