If a user is not logged in, they will see a prompt on the Quest panel that says "login to do Quests." As an admin, you have the option of adding a custom image above this prompt.
1. Add the quest panel.
2. Add the image where it says 'login image' by selecting from the media gallery or from your computer.
Once the image is added a user will see it before they log-in to do quests. The image will disappear once they are logged in.