The following screenshot shows a large banner promoting Rail Maze developed by Spooky House Studios UG (haftungsbeschränkt):
Your game will need promotion. One of the traditional ways is the graphic banners various advertising systems show. They can be created from the graphics your game is using: main characters, props, backgrounds, bonuses, and so on. All can be used. Don't forget about the text; short and memorable quotes from your game concept and plot are good.
There are a lot of banner formats in both landscape and portrait orientation modes. Some of them look like graphic stripes, while others can be displayed in full screen. Some of the popular formats are: 300 x 250, 468 x 60, 728 x 90, 640 x 960, and 640 x 100. I would recommend that you create the Adobe Illustrator files with several artboards and some basic artwork to export banners.