The GIMP (The GNU Image Manipulation Program) is a powerful image editing application. It will happily open and save to most image file formats and offers a host of useful features, plus the scope to add more using its plug-in system. Filters such as Motion Blur, Cubism and Predator make advanced image editing fun and easy. The GIMP opens several windows by default, which act as 'toolboxes' from which you can tweak settings and select tools to work on your image with.
GIMP can be found in the 'Graphcs' filter in the Application lens in the Dash. Alternatively, it can be run through the command line via:
From external PPA
The latest version of the GIMP could not be available in the default Ubuntu repositories. To install it, add the ppa:otto-kesselgulasch/gimp repository, through the Software & Updates app or via the command line:
sudo apt-add-repository ppa:otto-kesselgulasch/gimp
The GIMP may be seen as feature heavy for simple image editing. An example could be red eye removal on photographs. An alternative application for such photo editing is Shotwell, which is pre-installed in Ubuntu.
Official GIMP Documentation - Official documentation
Official Gimp Website - Official GIMP website
GIMP Wikipedia Article - Wikipedia entry for GIMP