The present invention is directed to a method of forming a positive image in a photothermographic film exposed by electrons in a transmission electron microscope to form a latent image in the film. The photothermographic film has at least one imaging layer comprising a potentially negative-working emulsion, but wherein thermal development of unexposed silver salts in exposed areas relative to unexposed areas is inhibited when thermally developing the imagewise exposed assembly, thereby producing a positive image. The present invention is also directed to the processing of the photothermographic film in which a positive image characterized by high speed and discrimination is formed in the film when heated above 150° C.