Take Pixelmator for example: http://appstore.com/mac/pixelmator
How do they have a background image set?
As far as I know, you have to be specially selected by Apple in order to do this; it's not something all developers can do unfortunately.
来源：https://stackoverflow.com/questions/16525408/how-do-you-set-a-background-image-for-your-app-listing-in-the-mac-app-store