I just upgraded to Sequoia 15 on my MacBook Pro 2021 and my desktop picture was replaced by trees. I was having the same problem with trying to get my preferred desktop wallpaper back. I did more research, found this article Customize the wallpaper on your Mac - Apple Support (CA) and discovered that you can drag a photo from a folder on your computer onto the thumbnail at the top of the Wallpaper settings page. So, go to System Settings, then Wallpaper, then drag & drop your photo into the box at the top of the window. Worked for me!
I was not able to get the "System Settings" --> "Wallpaper" --> "Add photo" --> "Choose" option to work. I could choose a photo from my files, but it didn't become my desktop wallpaper. It also doesn't appear to work to drag and drop a photo from Photos directly into the Wallpaper thumbnail box, but you can drag it from Photos to the desktop, then drag it from the desktop into the Wallpaper thumbnail.
I hope this works for the others who are having trouble.