I try to use my legs and hips to throw them to the side, face down. Its works sometimes, but if it doesn't they end us usually in side control.
Also, if that picture is accurate, push his face to the side and try to swing the outside leg over so you have both legs on the same side of his head and push down hard. You'll end up taking his back if you roll up on him fast enough...again if you miss, welcome to side control...which is what usually happens when you get stacked anyway. And if you can control an arm while doing it, that should set up a descent armbar, maybe an omo plata if you're slick.
I guess doing a bacwards roll would also get you to the turtle position...but I think thats kind of difficult with him squishing you if he moves well on the ground.