An app changed the wallpaper on my lock screen. I was so mad at the app, I uninstalled it.
I'd like to go back to the default wallpaper, but I don't remember what the default wallpaper was.
If it makes a difference, my phone is a Nokia Lumia 710.
To change the wallpaper (back), you simply go to Settings > lock+wallpaper, and choose the change wallpaper button.
The default wallpapers will show in a folder called "Wallpapers" (which won't show via the Pictures Hub). These will include a mix of any operator customised wallpapers, any manufacturer customised wallpapers and those from Microsoft. On my HTC device, this is a set of 40 to choose from.
The wallpaper on my device was the forth up from the bottom, on the right; but on my wife's, hers was the third down on the left. Another device of the same make/model I have access to used the third up on the left. Given this, I'm led to believe that there is no single default, but one is selected at random at some point during setup.
The default wallpaper on your phone can be set by nokia or your carrier/network so it is probably different for most people.
There should be a wallpapers folder in your photos hub so it will be one of those.
I also don't think an app can change a user's wallpaper but that's not a different story...