What property of ionic compounds explains why silver halides can be used in photography?

Silver halides absorb light to form elemental silver. This is the basis for the photographic film. This is an unusual reaction and is certainly not common to all ionic compounds. Silver halides are also unusually insoluble, again not a common property of ionic compounds. In fact silver halides have quite a lot of covalent character! I am sure teacher has something in mind but its certainly not in mine.