Hello Yanshi Chauhan,
A possible reason for this is that the image file was moved, renamed, or deleted from its original location after being set as the wallpaper. Windows 11 references the file path directly rather than copying the file, so if the file is no longer where it was, Windows may silently fall back to the default wallpaper.
It may be worth checking:
- That the image file is in a stable, permanent location (e.g. a dedicated folder in Pictures that you won't reorganize)
- Then re-apply it via Settings > Personalization > Background > Picture and browse to that path
Also worth double-checking:
- Background type is set to Picture, not Slideshow or Windows Spotlight
- If the device is signed into a work or school account, a policy may be overriding your wallpaper. In that case, it would be outside your control.