Hi Marion!
Click on the box up in the right hand side of your screen, where it says My Mood. A drop down menu will appear and you can change it there.
An avatar is any picture you have that will fit the requirements set as far as size goes. You can do this by going in to your user CP ( blue line above the Stumble INN sign on the screen, it has various options listed on it) and editing your user profile and adding a pic.
Hope this helps!