GIMP

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.