Please note that the whole 2.7.x series of versions is considered unstable and is not recommended for use in production even though it might just work for you. The most visible changes in 2.7.3 are the fully working single-window mode, including working session management, and the introduction of a new hybrid spinbutton/scale widget which takes less space in dockable dialogs.
To install GIMP 2.7.3 open a terminal window (press Ctrl+Alt+T) and copy+paste the following line:
sudo add-apt-repository ppa:matthaeus123/mrw-gimp-svn
sudo apt-get update
sudo apt-get install gimp
If you already have GIMP installed… just add the ppa and upgrade GIMP using the Ubuntu Software Center or the Update Manager. If you get an error message about a broken GIMP package that can’t be installed. You can fix this by upgrading the GIMP package using the Synaptic Package Manager.
GIMP Homepage