Background image in lock screen on Lollipop Upgrading to Lollipop, at some point the lock screen got a background image (see file attached). I never selected such image, and I can't find how to change in in the new settings menu :-/
Last but not least, the Lollipop lock screen shows full-screen still
  images from multimedia content being played from the device -- so you
  might see a graphic from a TV show or cover art from an album,
  depending on what you're streaming. In my experience, however, the
  images don't seem to clear properly when the content is no longer
  being played. I frequently saw images from a show I had stopped
  streaming long ago -- both when something different was being streamed
  and when the streaming app was no longer even active.


Found at Android 5.0 deep-dive review: Exploring Lollipop's many layers

In order to fix it I launched some streaming audio and locked the screen to verify the image changed.  Then I unlocked the screen and stopped the stream.  Locked the screen and the image was set back to default wallpaper.