Backgrounds

Just a quick question, I’m converting an old website to WordPress and would like to know the best way of doing the following,

The current site has multiple pages and the user used to have the option when creating a page to use either the red or the blue background tile. The backgrounds lived in the theme folder so they couldn’t upload any new backgrounds.

All they require is on the page edit/creation page a dropdown/select box to pick which background to use.

Could someone please suggest the best way to do this.

Thanks,

Tim