Paddle2See wrote:

The thumbnail image is taken from whatever was showing on the stage the last time the project was saved. So, run the project from within the editor until you get the image you want on the stage, then save it and exit the project. I usually make a screenshot of the stage the way I would like it to appear on the thumbnail, then add that screenshot to the project as a backdrop or sprite costume and then set up a way to display it so I can easily set the thumbnail every time I edit the project.

when green flag clicked

show list [ Thank You v]

add [Thank You!] to [ Thank You v]