Continuity mistake: When Billy and Brenda are arguing in her apartment towards the start of the episode, the pendant on Brenda's necklace moves itself back and forth between shots.
Continuity mistake: When Brenda and Billy are arguing in the casino, the shoulder strap of Brenda's handbag moves from tucked under her collar in the close ups, to sitting halfway along her shoulder in the long shots.
Answer: Nate actually say "You can't take a picture of this, it's already gone." It's kinda hard to explain, but he's referring to the fact that the moment Claire wants to take a picture of is over already, and she can't take a photo of it, just a different, lesser moment.