Thermal edge effects for crack-like defects have been calculated using the Wiener-Hopf technique. It is found that the crack surface temperature is reduced over a distance of a thermal diffusion length by heat flow around the crack edge to the cold underside of the crack. Surface thermal images have been computed for a straight-edged and a circular crack. Comparisons are presented of image profiles computed with and without inclusion of edge effects. Edge effects lead to the prediction that transient thermographic defect images should shrink with elapsed time. Experimental data are presented, which are in agreement with this prediction and demonstrate a simple means of defect sizing.