The main characteristics of a subterahertz image detector are calculated by numerical simulation of the thermophysical processes in a bolometric subterahertz-to-infrared converter based on a thinfilm metal-dielectric metastructure. The output signal of such a converter is recorded with an infrared camera and is used for thermal imaging. Simulations show that the energy conversion efficiency is over 80%, which ensures a high sensitivity of the imager and puts it in line with modern infrared detectors. The converter sensitivity is a linear function of the subterahertz radiation power in about three orders of the dynamic range. The real-time operation is provided when the dielectric layer is less than 3-μm thick. The blooming effect and the ways of its reducing are also discussed.