screen shot is black when the instance is a remote pc

I use webpagetest2.7, and instance is in a remote pc.

I’ve seen the answer from http://www.webpagetest.org/forums/showthread.php?tid=835

First I set the instance computer login automatically, then I restart the instance and do not login.

But I still got the black picture.

Can some one tell me, is there anything i’m missing?

Help!!!

Do you have the screen saver and power management disabled as well?