It's time to talk a bit about a very useful Blender tool: the Quick Edit tool.
Through this tool, it's possible to export a screenshot of the model in our favorite 2D painting software (Gimp or Photoshop, or whatever), paint on it using a new alpha background layer, and reassign the painted layer to the model in Blender, which is UV-mapped on the selected UV coordinates layer. All this, in just a few clicks.
In our case, we don't actually need to use this tool to refine the textures for the Gidiosaurus, so this recipe is going to be just an example. By the way, to fully understand how to use the tool, I suggest you to follow all the steps; just don't save the file at the end (or save it with a different name in a different directory if you want to keep it). So, carry on with the following:
Start Blender and call the User Preferences panel (Ctrl + Alt + U); go to the File tab and, in the Image Editor slot (Path to an image editor), write the path to your 2D image...