This is the starting image:

Code Geass Holiday Card First Image

Then you read this image:

Code Geass Holiday Card Image 2

The joke is is that it is supposed to be a Geass command.
And anyone who receives a Geass Command from Lelouch has to obey him.
His Geass is “absolute obedience.” So they have no choice.

See this article http://codegeass.wikia.com/wiki/Geass#Lelouch.27s_Geass

Here is an English dub clip:

Click here to view the embedded video.

And the full section in Japanese:

Click here to view the embedded video.

This is more of fan humor, but it works.