Comment 1 for bug 549268

Thank you for taking the time to make Ubuntu better. Since what you submitted is not really a bug, or a problem, but rather an idea to improve Ubuntu, you are invited to post your idea in Ubuntu Brainstorm at http://brainstorm.ubuntu.com/ where it can be discussed, voted by the community and reviewed by developers. Thanks for taking the time to share your opinion!

It is not something which can be fixed in the theme as well. There is no option to specify wallpapers.