How do I make non-garbage thumbnails?

So while browsing through the games, I find a lot of really nice looking thumbnails. Comparing my own to them I can't undestand why anyone would click on it. Do you have any tips, how a non-artist can make a somewhat appealing image for the game? What should I write on it and how? Should I use a screenshot from the game or do something from scratch? And if, how to do it, that it doesn't like like made with MS paint? Any advice and discussion is highly welcome! MainScreenshot.png