Board Thread:Support Requests - Designing Your Wiki/@comment-36633071-20180818193429/@comment-1038387-20180819072944

You can change your avatar by going to your profile and clicking "edit" in the masthead. See Help:Avatars.

There really isn't such a thing like a wallpaper. The background is set by the wiki, and though there is code to override that on your profile, noone else will see that.