Saving a background to a profile.

The desktop background selection is set in the registry in HKEY_CURRENT_USER\ControlPanel\Desktop\Wallpaper and ..\WallpaperStyle.

The simplest way to change the setting for Background is from the Control Panel. You would be able to modify the registry remotely if you know what you are doing in Regedit/Regedit32. The more common way in large shops is to use a desktop management application like Active Directory or Zenworks and push registry changes to the workstations based on user groups and other selection criteria.

If you are handy with login scripts, you should be able to export the relevant registry key to a .reg file and push that file to the workstation(s), even without Active Directory.

You can prevent the users from changing the background through Group Policies (gpedit.msc). The setting is in User Configuration - Administrative Templates - Control Panel - Display - Hide Desktop Tab.