Gosh, you have so much to be proud of. The game may not have a scripted "ending" but it has a full gameplay loop, and an intricate one at that. You even have procedurally generated levels. You accomplished more than 90% of the participants here.
No need for super fancy screenshots; I'd get one that shows the blue movement grid, cause that looked really nice, and one that captures the gunfire (from either side) could be cool too. I'd aim for 3 screenshots, and then honestly just re-use one of them with your game's title photoshopped over it for your cover image. Or use the design for one of your characters in the cover image, because their style and outline shading is very appealing.
Obviously, it's your page; do as you will. But I think the 10 days of effort you put into this game deserve to be validated with a few extra minutes of effort on your page. Good luck!