I assume you mean this auto-login account is limited, and for that reason you have no access. The best way is to temporarily add this user account to the Administrators group, then login with this user, make the changes, log out and back into the administrator account to remove the user from the Administrators group.
Best Answer
Create a new folder
C:\Users\<username>\Documents\background
Add
background.html
and yourbackground.png
Insert the following into
background.html
:<html>
<img src="background.png" alt="Selfhtml">
</html>
Open
background.html
with FirefoxRight-click on the image
-->
Set as backgroundVoilĂ , your result: