Definitely take pictures as evidence and call your local animal control immediately. You can call the sherrifs office and they can put you through to animal right away.
It even would be easier to actually GO to the sherrifs office in person, show them the pictures, and basically say this is whats going on and what are you going to do about it.
I had to do this a couple years ago, these people had a similar situation as you, but they had 33 horses on less than an acre of land. It was terrible. We ended up taking 3 of them (1 of them eventually had to be put down because of a terrible respitory infection and the other 2 were adopted into good homes.)
It took a little bit of bothering on my end but eventually the sherrifs department did something about it. And I feel very proud of myself for saving those horses that are in need. They all went to good rescue homes and got some major TLC and then were adopted out.