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We have investigated power handling of high-temperature superconducting microstrip filters employing highly sensitive thermal imaging. At low power, the images show small heating effects localized to areas with high current densities (ΔT<0.5 K), consistent with the known surface resistance and simulations of the current distribution. The breakdown at high-power levels, however, is always nucleated by a frequency independent local hot spot (T≳Tc). Scanning electron microscopy analysis of this area reveals a flaw in the superconducting material, presumably reducing the local critical current. The maximum power handling of these filters is, therefore, still open to improvement.