GIMP is a professional level image processing software package, that
- is completely free
- has amazing array of functions and filters
- is generally at same level as Photoshop
GIMP has lots of plug-ins available and nothing stops you to code you own plug-ins, so you can extend it almost indefinitely.
Unfortunately, it is not part of standard Ubuntu installation anymore, but installing it is very easy
sudo apt-get install gimp gimp-data-extras
You can find lots of free plug-in from GIMP Plug-in Registry. Usually you have to install them by hand, sometimes compiling from source or using Debian package installer.