what in the pic is nothing, its leaking from the core its self. it looks like a small radiator, and coolant flows through it, its how the heater works in the car.
you can also buy a pressure tester, it goes in place of the rad cap, and you pump it up to so many psi, then go looking for the leak, at this point it would be a good idea to have the carpet pulled up so you can see a active leak.
heater cores a bit of a pita to do, but can be done in a few hours. lots of little screws holding layers together.
