To use a single wallpaper and stop it changing at sign-in in Windows 11:
- Open Settings > Personalization > Background.
- Under Personalize your background, select Picture.
- Choose one image, or select Browse to pick a file.
- Avoid Slideshow and Windows spotlight, because those change the background automatically.
If the background is still changing, check whether the Bing Wallpaper app is installed:
- Open Settings > Apps > Installed apps.
- Search for Bing Wallpaper.
- If it is installed, uninstall it, because it can override the desktop background.
For stock Windows wallpaper images, one documented location for default Windows background images is:
C:\Windows\Web\Wallpaper
That folder contains default Windows backgrounds that can be selected manually.
References: