Apple - How do I take a picture of the login screen?

You can SSH into another Mac and use screencapture in the shell.

1. Enable remote login in the sharing preferences on the other computer
2. ssh username@other_computers_ip
3. sudo screencapture ~/Desktop/screenshot.png

I have not tested this myself, but maybe this link will help. Grab 'impossible' screenshots

OS X includes a nice command-line screen capture utility named, simply enough, screencapture. While there’s not a ton of documentation on this program, man screencapture will give you the basic options. Once you’ve used ssh to connect to the Mac whose screen you wish to capture, you need to execute the screencapture command with root privileges.