Yet another useful group policy at:
Computer Config – Policies – Administrative Template – Control Panel – Personalization
Force a specific default lock screen (See note below about creating file)
and
Prevent changing lock screen image

Note: Remember to create the image and save it to something like:
c:\windows\web\screen\williamslogin.jpg
Then when you enable the force policy you need to point to the file above.

This is specifically useful in the lab environment to give people information BEFORE they log in.

For details see:
http://www.grouppolicy.biz/2012/11/how-to-use-group-policy-to-change-the-default-lock-screen-image-in-windows-8/