For my game I have created a concept piece for what the cover of my game could look like, it is not perfect, however it is a start which I could improve upon later down the line and eventually get a very high quality cover for my game.
To create it I pasted my 2 character concepts into the page to show the viewer that this is what it is about and this is what they should expect when they play it, I also created a castle in addition to the two small platforming areas which highlights this game as a platformer. Finally I added the title and gave it a bold font and then added some clouds to clear up some white space and make the cover look a bit more lively.