There are probably other ways but I am just pushing a DMG from Casper of the following.
/System/Library/CoreServices/DefaultDesktop.jpg I doesn't prevent users from changing it, but it seems to apply to all new user on a machine.
If your wanting to force users to always use a specific background it probably wouldn't do it, although you might be able to find where the user equivalent is and have it apply a new jpg every login... slow and messy though, or alternatively push a launchagent that did it all client side every login.
Navigate to Computers > Configuration Profiles and create a new Profile. Then set the Profile to be User specific and create a "Restrictions" payload. Click "Desktop" and Lock the background to whatever you want. You could save the picture you want in a folder like "/.company/background.jpg" and use that as the path.