GIMP

Gimp is a powerful program to graphics processing. The most important is that it is completely free. Its capabilities are comparable to those, which can offer Adobe Photoshop. Allows on editing of all graphic files. You can use it to pictures processing, creating your own graphics, with use of extensive system effects. If it is too little, you can always go to the extra (also free) plug-ins written by programmers, which keep you updated with it. You can say that the possibilities of Gimp are unlimited. The thing, which can be a problem, is a different approach to organizing the working space, than it was in Windows (although there are plugins, which transform the Gimp on a product more accessible for windows users).