This is true that Adobe Photoshop is king in photo editing world but his is not only one in this field. You can find out top 9 and best alternatives of Adobe Photoshop in this post.
Gimp works across several different platform (Linux, Mac, Windows, FreeBSD, Sun OpenSolaris) and is perhaps one of the best free alternatives out there so far. Gimp has a large user community, with great list of tutorials on the official sites.
[Windows only] This photo editor runs on Windows. It has a large and growing community of users, with lots of tutorials and plugins.
[Windows only] This free photo editor originates from Japan, with English versions to download. Supports layers, masks, and some basic tools of Photoshop.
[Mac only] Seashore is an open source image editor for Mac OS X’s Cocoa framework. It features gradients, textures and anti-aliasing for both text and brush strokes. It supports multiple layers and alpha channel editing. It is based around the GIMP’s technology and uses the same native file format.
Inkscape is an open-source vector graphics editor similar to Adobe Illustrator, Corel Draw, Freehand, or Xara X. What sets Inkscape apart is its use of Scalable Vector Graphics (SVG), an open XML-based W3C standard, as the native format.
Xara Xtreme for Linux is a powerful, general purpose graphics program for Unix platforms including Linux, FreeBSD and (in development) OS-X. Formely known as Xara LX, it is based on Xara Xtreme for Windows, which is the fastest graphics program available, period.
[Mac only] ChocoFlop is an image editor for Mac OS X, based on the CoreImage technology. It is currently in beta stage and available freely.
[Windows only] PhotoPlus is the amazing photo editing software that enables you to fix and enhance digital photos, create stunning bitmap graphics and even web animations.
CinePaint is a deep paint image retouching tool that supports higher color fidelity than ordinary painting tools.