Kamishiho Gatame is a great control move in the sport application, but not really practical (IMHO) in a true jujitsu sense. What do I mean? Well, in that picture, the assailant is not using his body weight to smother the person on the ground, thus, he is really just imobilizing him.
Now, from a Jujitsu perspective, I would use the same movement with my legs as Joe and Rory perscribe, but I would add the ever fabulous, but oft forgotten, ATEMI (aka, distraction). The person on the bottom of that piture has both hands/arms free to reach his opponants head. I would call that free game at this point. Constant pressure to any of the millions of pressure points on the head would suffice to get them to move. You also have your chin to stick into his sternum. Once you get him on the move, then curl your legs, turn your hips and shrimp out of that position.
I would be more worried with someone who got me into Yoko gatame and was able to hold it.