I've never seen such a requirement myself**, and I stand to be corrected, but when the agent asks for a description of the characters, I think it's highly unlikely he or she wants to know height, weight, shoe size and colour of hair. What I should imagine is needed is a description of character -- ie what kind of person the hero is, what drives him, why he reacts as he does. In those circumstances, I'm not sure how a picture is likely to assist.
** Angry Robot -- the publishers -- for their open submission month want a "summary of characters", but that's certainly not a description of physical characteristics