The wallpaper is always whatever it comes with and never gets changed. Seemingly the one I'm using at the moment came without wallpaper and just has a blue background.
I thought I had the plain Blue windows background, just checked - It's a sort of pastel affair with an Acer logo. heh....only been using it for about 9 years!
But seriously ... I do change my wallpaper fairly often and, the one I've got at the moment on my desktop is a nice Autumnal red background with yellow and golden leaves which I quite like so it'll be staying put for now.
I used to, but the default wallpaper has done me for a good few years now. The built in Windows ones are naff and downloading any from the net bring a load of crap with them.
Have changed it at least monthly for the last few years, my own recent favourite photos, mainly steam train based until January, since then changed more or less weekly for latest pic of grandson.
Wallpaper on tablet & phone also changed regularly in similar style.