Topological insulators (TIs) exhibit a quantized magnetoelectric response when time-reversal symmetry is broken on its surface. This unusual electromagnetic (EM) response is a unique macroscopic manifestation of the quantum Hall effect on the TI surface and it is described by a quantized Chern-Simons theory. In this paper, we construct the Green's function (GF) describing the EM response of two topological media separated by a cylindrical interface. This GF, in the appropriate limits, describes the magnetoelectric effect of both i) a cylindrical TI surrounded by a dielectric fluid, and ii) a TI with a cylindrical dielectric-filled cavity. We calculate the EM fields produced by a line charge and a line current near the TI surface and we show that, in addition to the standard image electromagnetic sources, charge and current densities of magnetic monopoles will also appear. We discuss some experimental setups which could be used to test these magnetic monopole fields.