If you like a particular desktop wallpaper in Windows 7 and do not like anyone to change it, you have to do the following steps to prevent other users to change the desktop background.
Click the Start button, type “gpedit.msc” (without the quotation marks) in the search field and hit Enter from keyboard:
Windows 7 Group Policy Editor will open; on the left hand pane expand User configuration >Administrative Templates >Desktop >Desktop :
On the right hand pane, you will get settings for the Desktop Wallpaper :
Double click it to bring the Setting Window. Select “Enabled” under Desktop wallpaper and specify the location of the image or wallpaper that you want to make permanent in the field below it:
Click Apply and OK to confirm the changes.
Note: You will notice the changes after restating or logging off and again logging in to Windows 7.
Now no one can change your desktop background except you. To revert back, Just make the settings to “Not configured“.