During human-robot interaction, it is often necessary for human users to command the robot's locomotion. In this paper, we design an intuitive interaction mechanism for users to command a NAO robot's locomotion with predefined postures. Based on images taken by its onboard monocular RGB camera, the robot can localize the human user's head, torso and arms, and recognize the posture displayed by the user with the k-NN algorithm. Then the robot can move according to the locomotion command (forward/backward/left/right) associated with the posture. We test these postures in an HRI experiment and get good recognition performance.