We present hard X-ray imaging observations of impulsive solar flares made with Yohkoh HXT, with emphasis on the behavior of hard X-ray sources in the energy range above 30 keV. The bulk of hard X-rays in this energy range is emitted from the footpoints of a flaring magnetic loop by electrons accelerated near the top of the loop, manifesting itself most frequently as double sources located on either side of the magnetic neutral line. The temporal behavior of such hard X-ray double sources contains ample information on the evolution of particle acceleration (energy release) site as well as on the structure of the flaring magnetic field itself. By analyzing a total of 14 impulsive flares commenced between October 1991 and December 1994, we have found the following: For flares which show increasing footpoint separation, which may suggest loop-with-a-cusp-type magnetic field structure, super-hot plasmas tend to be created at the top of the flaring loop during the impulsive phase. On the other hand, flares without separating footpoints (or even decreasing separation), which could indicate emerging-flux-type magnetic field structure, tend to lack such super-hot plasmas, showing the double sources without any bridging structure between the two even in the lowest energy band of HXT (14-23 keV). We also found that the separating footpoints tend to move in an anti-parallel manner to each other, along (not perpendicular to) the magnetic neutral line, while the not-separating footpoints do not show such systematic motion. These observations will be presented in detail together with the corresponding soft X-ray and Ha images. Implications of the observations to particle acceleration and plasma heating in the global magnetic field structure will be discussed.