If you want to have a background image for your gallery, then there are essentially two different ways to achieve this.
(1) Use Juicebox-Pro's own Background Image Options.
The backgroundUrl can be an absolute URL (in the form http://www.example.com/image.jpg) or a relative URL (relative to the web page containing the gallery's embedding code).
The easiest thing to do is simply use the background image's filename as the Background Url (set in JuiceboxBuilder-Pro's 'Customize -> Background Image' section) and then copy the image directly inside the gallery folder (after saving the gallery on the 'Publish' tab) alongside the 'config.xml' file and 'index.html' file. (Do not add the image to the gallery on the 'Images' tab as this will display the image in the gallery.)
(2) Give the Juicebox gallery a transparent background and use CSS to set a background image for your gallery's container.
Assuming your gallery's container is named 'juicebox-container' (the default name), then set the Background Color Opacity to 0 (in JuiceboxBuilder-Pro's 'Customize -> Lite' section), add the following to the <head> section of your gallery's 'index.html' page (changing the image filename as necessary) and copy the image directly inside the gallery folder alongside the 'config.xml' file and 'index.html' file.
Suggestion #1 has the advantage that the custom background image will remain when the gallery is expanded.
I hope this helps.