The GIMP (The GNU Image Manipulation Program) is a powerful image editing application and can be installed* via the Ubuntu Software Centre in Ubuntu, or
sudo apt-get install gimp
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:
*The latest version of the GIMP (with single-window mode) - 2.8 - is not available in the default Ubuntu repositories. To install the new version, add the ppa:otto-kesselgulasch/gimp repository, either through the Software Sources app in Ubuntu Software Centre > Edit > Software Sources. Or add it via the command line:
sudo apt-add-repository ppa:otto-kesselgulasch/gimp
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 F-Spot.
Official GIMP Documentation - Official documentation
Official Gimp Website - Official GIMP website
GIMP Wikipedia Article - Wikipedia entry for GIMP