The following compound sentence will allow you to take screenshots of basically any remote Mac which has SSH running and passwordless SSH login set up:
ssh mymac "echo 'adminpass' | sudo -S screencapture /tmp/screen.png && sudo sips --resampleWidth 400 > /dev/null 2>&1 /tmp/screen.png && cat /tmp/screen.png; sudo rm /tmp/screen.png" > screen.png && open screen.png
The sudo is necessary to be able to make screenshots from a “windowless” environment.
One of these days I will integrate this into a widget which will look just like the ARD widget (guess where I got the idea for this) and read the remote pass from the Mac OS keychain. I would have liked to call it FarSight, but since a piece of software by that name already exists, I will just humbly call it SuperView. ;)