Time and light. The two factors involved in exposure, both of which can create and destroy an image. Place a photograph in bright light and it will slowly fade to nothing. The paradox of photography has always been that light can give and take away in equal measure.
I often forget how important photographs are to people. They are, in many cases, irreplaceable and priceless items. A connection to a time, place or person long gone. The older we get, the more we seem to cling onto them for comfort. Our memories are converted into a flat piece of photographic paper to be retained for future reference.
I once did a photo restoration job for an old lady whose only image of her mother was fading due to the photograph's age. The old Edwardian photograph was in it's final phase of fading and cracking due to the strain of nearly a hundred years of handling. I can still remember the look of happiness/relieve on her face. What is an image worth? More than we often care to realise!