Over the years many clients have sent in multiple pet snapshots to me at Art Paw of every dog in their household. I can almost always tell which dog is the boss, or Alpha in the pack. I often ask them about their pack order to check my guess work, and I am usually right. The boss dog is the one staring straight into the camera lens, ears perky and happy. The more passive members of the household will not stare at the camera and if they are really shy their ears will be down.
If you think about it this is just their survival instinct at play. You are the real boss in your home and the major Alpha in your pack, so keep in mind that dogs communicate a lot with body language. If you stand directly in front of them to shoot them you are confronting them head on and they are likely to look to the side so as not to offend you or challenge you. The Alpha dog in your pack may have enough confidence not to be too overly concerned, but more passive pups will really not be comfortable with you standing right in front of them. A much better approach is to move a little to the left and then try to get them to turn their head in your direction by making some funny noises.
Below are 2 of my Scotties. Can you tell which dog was Alpha? Big Tommy was our boss dog for many years. He was the most photogenic pup we have ever had. He was confident and unconcerned about the camera. We still manage to get some good shots of the other pups, but it takes a bit more work and patience.