The magnetization of a Co disk with an in-plane flux-closure domain structure was subject to a picosecond magnetic field pulse perpendicular to the plane. Images with sub-micron spatial resolution have been recorded every 10 ps which reveal magnetic non-uniformities of the ferromagnetic resonance (FMR) response. A detailed analysis of the images shows a difference in the FMR frequencies of about 10% as a function of radius. In addition, the magnetization at the edge responds earlier to the applied field pulse. We discuss possible origins of these phenomena.