This paper addresses the problem of visual control of a set of mobile robots. In our framework, the perception system consists of a calibrated flying camera looking downward to the mobile robots. The robots are assumed to undergo planar motion considering nonholonomic constraints. The goal of the task is to control the multi-robot system to a desired configuration relying solely on visual information given by the flying camera. The desired multi-robot configuration is defined with an image of the set of robots in that configuration. Then, any arbitrary configuration can be easily defined by this image without any additional information. As contribution, a new image-based control scheme is presented relying on the homography induced by the multi-robot system to lead the robots to the desired configuration. The stability of the control law is analyzed and simulations are provided to illustrate the proposal.