Where are app-provided wallpapers stored?
Check out this answer, just in case the wallpaper was moved

That being said, since you're looking for an app-provided wallpaper and not a system provided wallpaper, we'll probably need to know the package name of the application in question (and you'll need to have root access to your device, which I believe you already have). 

So assuming the package name of the app is com.example.superdupperlauncher, then you would need to look into /data/data/com.example.superdupperlauncher/files/ or /data/data/com.example.superdupperlauncher/