I don't understand your questions, are you looking for explanations on how to edit the picture, like what tool serves for what or are you asking for how to take a screenshot of your meez?
in case your question is indeed about how to use gimp on what are the tools functions and such, i highly advice you to check this :
Gimp Tutorial: Basics And More
this will not be the absolute solution and will not necessarily have the answer to a particular question , but you'll find a pretty good tut on how to use gimp.
also you can check Peny's sticky called "tutorial directory" where the most useful tutorials are listed, there's a section reserved to GImp so you'll definitely find great help
there