over a year ago:
Just because he recognized her, it doesn't mean that Elena is the reason why. He could have recognized Katherine, because in my opinion, he's still in love with her. So, wouldn't he be able to tell the difference? He's kissed and been around both girls, and there have to be some differences. Plus, he had that handy picture as a reminder, and I'm sure that brought back all the memories of what she looked, felt, and smelt like. And he knows what Elena's like too. So therefore, he had a better shot than Damon to recognize her.