GIMP is a free and open source powerful image editing program, on a similar level to photoshop. GIMP supports high quality image manipulation, original artwork creation, Graphical design elements for UI components and mockups, scripted image manipulation and support for community developed plugins.

The interface can be a little overwhelming to start with, but once you get used to it there are very few limits of what you can achieve with your artwork in GIMP.

• Check out a video by Davies Media Design giving a complete overview tutorial for Gimp to get started
• Get the tool here.