You need to add the background image to the HTML page in Dreamweaver. If al you want to do is add a background to your mockup, then add a white rectangle the same dimensions as your document, and move it to the back (Modify > Arrange > Send to Back. Once you've done that, enlarge the document window and add your background image. Move that to the back as well.
Thank you for responding. When I add the background in Dreamweaver, it covers the entire page, including the mockup. I would like to set the background so that it just covers the areas outside of the canvas. Is it something I'm doing wrong? Thanks again for your help, it's much appreciated.
I think I was able to solve the issue. When I set the background in the page properties panel, it automatically set it to cover the entire page. I had to mess around with the CSS to get it to only show on the outside. Thanks so much for your help!