Why does my home screen picture keep changing?

[AC Question]

My home screen picture keeps changing on its own, I have nothing weird installed on the phone, I haven't changed anything since I've had the phone. This just started recently. I can't seem to figure this out.

 

[B. Diddy]

Welcome to Android Central! Which phone? Are you sure you don't have a live wallpaper installed that changes the background periodically? Something like Muzei?