When I disconnect with the Remote Desktop Connection the screen shot for the webpagetest is blank.
Yep, check out the last item in the notes here: https://sites.google.com/a/webpagetest.org/docs/private-instances#TOC-Test-Machine-s-1
You need to reboot the machine when you are done with a RDP session, not disconnect. Otherwise the desktop gets locked and screen shots aren’t possible.