That's MRS. McFoxFace to you!
If you click on the image gallery, you will see a "more..." button under each image.
If you click that you should see "propose representative image"
Yes, just to clarify: The image that represents the game needs to go through the mod process. So, you propose the image that should represent the game and the mods will review it and certainly approve your suggestion.
This process is mostly in place for games which have hundreds of images and it can be a debate over which one should represent the game in the search.