Press space to see how to make a thumbnail! To test out if it works you have to press the stop sign (the thing right next to the flag) it should show your thumbnail... Sorry about the white dots all around the script I just couldn't make them transparent! I'll probably use thumbnails from now on ;) Another way you can make a thumbnail is that you can just add the "When Flag Clicked" and the "Hide" block underneath and it should work, but don't take my word for it...
This is my first "How To" project. I had just recently learned how to make a thumbnail! If it doesn't work then you can message me on my profile or write a comment on this project!