» How do I change the background image?
C5 Extras > Support > Knowledge Base > Express Theme > How do I change the background image?

The background image is a Page Attribute which is page property and unique per page. To assign the background image to one of the pages in Express theme all you need to do is go to Edit > Properties and then a popup should be displayed. Once you see the Page Properties window go to Custom Attributes tab and then search for the "Express Background Image" and assign the image that you want to use.