You could do what i do........run around like crazy with a lunge whip in one hand and my camera in the other! LOL
Of course, it's difficult, I have to run, then stop, focus my camera and *hope* my mare is still moving out like I want her to. By the time i get it focused, she usually has stopped and started grazing again
FORTUNATELY, I have gotten some pretty darn good pictures doing this!
If you are looking for halter pictures, try to teach your horse to pose and stand still when you are not able to hold them for the picture. I can walk up to my mare in the pasture, snap my fingers at her legs and point or tell her which ones to move. Getting her to stand posed is fairly easy, the only more difficult part is getting them to do the right thing with their neck when you arent right there...and of course, the first biggest issue is making it so they don't walk off.
Kind of OT, but with my Shetland Sheepdogs, I can tie them to something, somewhat pose them and toss things in front of them or use squeeky toys to bring there attention where it's needed. This is also something you could do with your horse.
Unfortunately, it's not easy getting/taking good pictures of horses by yourself, I have lucked out a lot on gotten pretty good ones, it just takes a lot of practice., and a LOT of pictures