A person riding on a dirt bike while someone takes a picture.
There is a person.
There is one person riding a bike and another taking a pictures, so there is at least one person.
exists e1 p1 x1 x2 x3.(exists e2 x4 x5.(Theme(e2,x5) & Actor(e2,x4) & v_take(e2) & n_picture(x5) & n_person(x4)) & while(e1,p1) & on(e1,x2) & n_bike(x2) & of(x2,x3) & n_dirt(x3) & Actor(e1,x1) & v_riding(e1) & n_person(x1))
exists p1 x1 x2.((x1 = x2) & n_person(x2))