1. Have enough different photos for 4 covers.
2. Have the right looking couple for both male and female.
3. Have the right emotion.
Usually you might be lucky and find 1/3, occasionally 2/3, but 3/3 is practically unheard of. Doing a dedicated shoot means that you can hit all three much easier.
I still don't know that we got the covers for all of the books, mostly because the day is just a blur of excitement now, and the photographer is now working her magic. The photos I have seen though - GAH it was perfection. It's one thing to imagine what it might be like to have your own shoot down, to imagine people actually embodying the characters you've created. Another entirely to see it play out in front of your eyes. I imagine it must be what authors feel like when their books are made into movies. Maybe that's the next step, huh?
I'll leave you with some brief sneaky peeks: