A part of me thinks this may have been mentioned on WW in the past, but I couldn’t find any mention in a cursory search.
Check out Microsoft | Creators | WallpaperHub for a repository of Microsoft’s desktop backgrounds. Many of the ones featured by Bing are there, along with some “unreleased” ones for Surface and other products.
One neat thing is they offer many images in resolutions classified by Surface products. Nice to have when you’re living in a 16:9 world with a 4:3 display!
The site is run by a Microsoft Insider named Michael Gillet. He along with a few other artists have their creations on the site as well. The Insider blog did a writeup on him last month - Meet the Insider designing your favorite new wallpaper (windows.com)
Currently using the 6 year anniversary image on my Surface laptop, and I have some of the fluent designs cycling around as my terminal app background. Feel free to share your favs!