Don't know yet but might if you take a moment and tell us what "Norton software" you are referring to and what Operating System you are using.
It is likely a change in a configuration setting of the Norton Product caused by the "download from my Norton software" (an update we must presume).
Also tell us step by step how you personally are trying to change the wallpaper as there are more than a couple ways to do that.
I have always used pictures of my grandchildren for my wallpaper but after I received a download from my Norton software I am unable to change my background to pictures. I can change the color of the background but cannot click on the pictures including those choices that are on my computer. What has frozen the pictures?