This paper shows how to push an unknown object in the plane from an initial pose to a target pose with two cooperating mobile robots. On the object motion, we deliberately impose non-holonomic velocity constraint with pushing mobile robots. This yields smooth and efficient trajectories. Pushing manipulation is performed, for the first time, with a new uncalibrated image-based control scheme. This is achieved by transforming the image information to a scaled Euclidean space without using any metric information or calibration. Stability of the control law is also demonstrated.