First off for just registration pics or transfer pics, they do not need to be anything fancy. The association is just wanting to make sure it is the same horse.
For professional ad pics here are my tips- Morning is best as everyone else is saying. You need at least someone to take the pictures, and at least two helpers, one to hold the horse and one to get the horses attention. You will need plenty of "attention getters" such as umbrella, large mirrors, glittery magic wand, breath spray, Baby powder (spray some in the air), shiny balloon, big stuffed animals, grain buckets, electronic flashing toys, etc..... It does help to have a good camera. I have a Canon Rebel and use all the cheater settings. Plan to take 5-100 pictures per horse to get a few special ones. Make sure horse is clipped and oiled up, just as if you are going in the show pen. Have a nice show halter on and make sure it is fitted correctly. You need to be at "horse level" and with minis that is pretty low. They say to get a profile shot (side), you need to stand on the side of the horse, closer to the hip. Fill the lens with as much of the horse you can, do not have a lot of scenery. Focus on the middle of the horse when you take pictures.
This is from a photo shoot I did on my yearling colt last year. I am not a professional photographer but I have a friend who is