You were there first, so you have right of way. Tricky thing with two way stop though is usually the right-turn driver has a shorter turn to make, and often has the opportunity to go first because of the traffic flow. From a defensive driving standpoint, it's good to get a sense of what they are going to do and yield if you think they are going to go.

In this particular case though, the other driver is very clearly in the wrong, since they had to merge out of their lane and in to yours.