Alot of horses are branded. Most states require a registration on the brand. I would start by calling up the State brand inspector. There are also brand sights online that you can post it on. But again I would check with the state brand inspector you can call them and describe it over the phone and they can do a computer search to see if it is listed in the last 10 years. If you can't find it then you might have to expand your search to other states.
I'm the paint owner from New York. I recently found and confirmed that my horse was born and branded in Nebraska. How he got from Nebraska to New York is my next mission!. Anyway, I will be glad to help you search. What country do you live in? Is there a way to get a closer picture like this one?