Scold the employee right back, tell them you are taking pictures for a friend, or better yet, a parent who wants something but they don't know what...
Then tell them that you will go to their competitor because they are rude!
You could also add that you are standing on public property (assuming these are the drive thru board ones) and why is it a problem for them that you are taking a picture to gather order information!!
If you are not inside the establishment, there is no legal problem with taking pictures. Tell the employee to get the manager and speak to him/her.
Of course this is not optimal for a MYSTERY shop but I will tell you right now, no fast food employee is going to yell at me for taking pictures OUTSIDE the establishment.
O.o o.O
Happily shopping New England and beyond!!!!!