  Observing the motion of the nuclear wavepackets during a molecular reaction,
in both space and time, is crucial for understanding and controlling the
outcome of photoinduced chemical reactions. We have imaged the motion of a
vibrational wavepacket in isolated iodine molecules using ultrafast electron
diffraction with relativistic electrons. The time-varying interatomic distance
was measured with a precision 0.07 {\AA} and temporal resolution of 230 fs
full-width at half-maximum (FWHM). The method is not only sensitive to the
position but also the shape of the nuclear wavepacket.
