Why is it possible to image LUMO if these orbitals are, by definition, unoccupied?
Why not? It's not like we poke them with our finger, anyway. All we can really see are the electron transfers. Now, the transfers from the molecule are as good as the transfers to it. If we adjust our setup so as to look at the latter, then we may see the shape of the orbital which is now empty but ready to accommodate our electron.

In one setup, the electrons tunnel from the HOMO to the tip, thus visualizing the HOMO. In another setup, they tunnel in reverse direction (hence the change in the sign of voltage), from the tip to the LUMO, thus visualizing the LUMO.