Generally you would use CSS to add a background image to the story area of the current page, and that CSS would be placed within your project's Story Stylesheet area.
background-image: url('the URL of the image you want shown');
If you want to use Passage Tags to control which background image is currently shown then you need to change the CSS selector to include the name of the relevant passage tag.
eg. If you want all passages that have been assign the forest Passage Tag to have the "media/forest.png" file as the page's background you would use a CSS selector & rule like the following.