  A galaxy halo may contain a large number of intermediate mass black holes
(IMBHs) with masses in the range of 10^{2-6} solar mass. 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 ~ 1 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 (< 0.3 mas) will be
capable of directly mapping such off-nuclear freely roaming IMBHs with a mass
of ~ 10^6 solar mass in a lensing galaxy that harbours a O(10^9) solar mass
supermassive black hole in its nucleus.
