Gimp has been my favourite image editor for many years. It’s easy to use and simple to install. You can install Gimp by adding the otto06217 PPA.
GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of software for such tasks as photo retouching, image composition and image authoring. It works on many operating systems, in many languages. Read more about Gimp.
To add the PPA and install Gimp open a terminal window (press Ctrl+Alt+T) and copy+paste the following lines:
sudo add-apt-repository ppa:otto-kesselgulasch/gimp -y
sudo apt-get update && sudo apt-get install gimp -y
When installed close the terminal window and to start Gimp press the SuperKey and type: gimp