Set an action in photoshop - this will remember the process you have done therefore you dont need to keep re-doing everything.
1. Window > Actions
2. Click record and name your action.
3. Select > All
4. Edit > Stroke (40px, black)
5. Select > Modify > Contract (Half pixels of last choice, so my example i'd pick 20px)
6. Edit > Stroke (2px, white)
And there is your border!
Remember to do this as an action, saves a vast amount of time