All photographs are memento mori. To take a photograph is to participate in another person’s (or thing’s) mortality, vulnerability, mutability. Precisely by slicing out this moment and freezing it, all photographs testify to time’s relentless melt.
Or, as Mitch Hedberg put it:
One time, this guy handed me a picture of him, he said,"Here's a picture of me when I was younger." Every picture is of you when you were younger. "Here's a picture of me when I'm older." "You son-of-a-bitch! How'd you pull that off? Lemme see that camera...what's it look like?"
Btw, Buzzfeed posted A Complete Ranking Of (Almost) Every Single Mitch Hedberg Joke.