Hello,
I'd be happy to add the background image for you!
Do you have a direct link to the site, please? I'd like to check to see if a child theme should be created as part of this project. A child theme may be important... if it's a commercial theme and an update is available, the update would overwrite the changes in this project. If that doesn't make sense, I'd be happy to explain!
Thanks, and hope to be working with you!
- John