I would like to add some copyright text to the footer of the page
I don't know how to do that, but you can add a copyright watermark to every photo and thumbnail in the gallery: Work with web gallery layouts in Lightroom
I am trying to find a way to set a default background image for my web galleries.
The moderator moved your message into the subforum for discussing how to implement LR plugins. I'm guessing you don't want to program your own Web gallery plugin -- this would accomplish your goals, but it requires strong programming experience and an investment of many, many hours. See the Lightroom SDK documentation for details: Adobe Photoshop Lightroom | Adobe Developer Connection.
If you're not interested in programming your own Web gallery plugin, you might repost in the main LR forum, prefaced by a plea to the moderators not to move the message into this forum.
Finally, you could good search the main LR forum and Google for recommended third-party LR Web gallery plugins. These provide a lot more flexibility than the built-in templates provided by LR.