A hidden message does say 'Where is Dr. Lamb?'
on the woman's chest, so I'm guessing it is her after all?
I don't think so. As you can see there is also a circle above Ryan's head probably symbolizing the place where who wrote this(most likely Lamb's followers) thinks Lamb's head should be.
And I don't think it is Lamb in general because first of all Lamb doesn't seem like the person to dress up like that. Also the woman on the poster looks like she has a much rounder face as well as eyes set more apart.