A galaxy halo may contain a large number of intermediate mass black holes ( IMBHs ) with masses in the range of 10 ^ { 2 - 6 } \textrm { M } _ { \odot } . We propose to directly detect these IMBHs by observing multiply imaged QSO-galaxy or galaxy-galaxy strong lens systems in the submillimeter bands with high angular resolution . The silhouette of an IMBH in the lensing galaxy halo would appear as either a monopole-like or a dipole-like variation at the scale of the Einstein radius against the Einstein ring of the dust-emitting region surrounding the QSO . We use a particle tagging technique to dynamically populate a Milky Way-sized dark matter halo with black holes , and show that the surface mass density and number density of IMBHs have power-law dependences on the distance from the center of the host halo if smoothed on a scale of \sim 1 \textrm { kpc } . Most of the black holes orbiting close to the center are freely roaming as they have lost their dark matter hosts during infall due to tidal stripping . Next generation submillimeter telescopes with high angular resolution ( \lesssim 0.3 mas ) will be capable of directly mapping such off-nuclear freely roaming IMBHs with a mass of \sim 10 ^ { 6 } \textrm { M } _ { \odot } in a lensing galaxy that harbours a O ( 10 ^ { 9 } ) \textrm { M } _ { \odot } supermassive black hole in its nucleus .