The image on an exposed and developed photography film is due to :

On one side, a strip or sheet of transparent plastic film base is covered with a gelatin emulsion that contains tiny silver halide crystals that are light-sensitive. The silver halide grains that have been exposed to light are turned into metallic silver during the development process, which gives the film's black area its appearance since it blocks light.
Ag is responsible for the picture on a photographic film that has been exposed and processed.