There are 2 options to choose from if you "see inside" How to add this to your project: click "see inside" and drag the Sprite to your backpack below. Add this to your project, and it should appear in the front. I have already added code to the sprite that will make it disappear once you start the project, however it will still have the thumbnail so that people will see it. Hope this helps!
The person who this thumbnail is for, is super cool and stuff. You should check them out (and play the first maze game!!)