Terahertz time-domain imaging (THz-TDI) has been applied for imaging a hidden portrait and other subsurfacecomposition layers of an 18th century (18C) easel painting by Nicolai Abildgaard, the most important 18CDanish neoclassical painter of historical and mythological subjects. For the first time, a real hidden portraiton an easel painting has been imaged by THz-TDI, with an unexpected richness of detail. THz C- andB-scans have been compared with images obtained by x-ray radiography and invasive cross-sectional imaging,leading to a deeper understanding of the strengths and limitations of this technique for art diagnostic purposesand defining its role among complementary tools for the investigation of art objects. We present a fast and effectivemethod to separate single THz pulse reflections of interest from the entire signal across the image, adaptedfor uneven surfaces typically encountered in practical applications of the technique. Interfaces between layers ofthe painting have been successfully imaged, contributing substantially to the understanding of the structure of the painting.