How do I delete Windows 8.1 my own account lock screen photo

I have a fairly new ASUS laptop with Windows 8.1 and have been playing around with the settings etc. and have a question regarding the lock screen.

I have personalized the lock screen with my own picture but have decided that I don't like it and would just prefer to use one of the default pictures, but there doesn't seem to be anyway of deleting the picture I have added.

I've tried searching for the picture on the laptop but have had no success finding it.

Can someone please advise me as to how delete this picture?

Many thanks,
Mike

Possibly useful link:

https://social.technet.microsoft.com...w8itprogeneral

Thanks! I managed to change my thumbnail "mug shot" with my dog's photo. I think that thumbnail was blank before took my photo.
Mike

After some Windows updates, I just noticed my very small thumbnail lock screen photo is layered on top of my dogs photo???

It's not a big deal but my thumbnail is somewhere so I can delete it.

Here are step by step screenshots + INSTRUCTION.
http://www.askvg.com/how-to-disable-...-in-windows-8/