How to Prevent user from Changing Wallpaper in Windows 7
Open Run Dialogue Box (Win + R) and type gpedit.msc.
Go to Local Computer policy > User Configuration > Administrative Templates > Desktop in left side pane.
Select Desktop Wallpaper setting from right side pane.
Double click it.
Click in the "Enable" Radio button and set the path of the wallpaper you want to display constantly.
Press OK to Save changes.