Android - Background image in lock screen on Lollipop

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.

Had the same issue. TuneIn radio is indeed the culprit. In order to get rid of it do the following:

1. Launch TuneIn
2. Play any channel
3. Lock your screen
4. Go to lock screen
5. Stop TuneIn from the lock screen (press square stop button on displayed widget).
6. Background reverts to normal. Done!