Can I get a photo with the pilots?
I don't know about the situation specifically in the US, but in my - mostly European - experience crew are happy to welcome you to their office after flight. 

When I feel the need to see the cockpit or discuss things with the pilots I try to ask the flightcrew before take-off if I can meet them after the flight. Sometimes I explain my interests to the cabin crew and often they will arrange the visit. In a few cases I just asked while deboarding.

Depending on the aircraft type and airport situation you have to think about the logistics a bit. In single aisle aircraft, if you start a discussion with crew while deboarding, you are blocking the queue. This gives the feeling that it is an inconvenient moment and reduces your chances. So if you ask at the last possible moment, make sure most people already exited before you. 

However, if passengers are transported to the terminal by bus, you will be holding the bus since you are the last to get off. This reduces the time you can spend in cockpit drastically. If you expect to be bussed to the terminal, ask cabin crew before the descent and don't wait until everybody has exited.