You can upload 1-3 images to your Cover Image Gallery. The first image (on the left) will also appear as your listing preview cover in search results. 

If uploading only 1 image, the image should be banner style to fit the entire width of the screen. 

Images with text are not permitted in the Cover Image Gallery, but can be added to your Bonus Image Gallery if your package includes this option. The subject matter should be as centered as possible to avoid being cut off at the edges.