InfraRed Thermal Imaging (IR) permits to non-invasively map the skin temperature distribution of the human body. Combining together thermal and visual imaging enriches the informative content of IR images and provides to the physician an anatomically-based and more friendly visualization--on the thermal images--of the presence of possible pathological processes. In this paper we use a homography technique to overlap a thermal image to a visible one. The technique provides an effective pixel-to-pixel correspondences between the two images or their portions.