Perhaps it sounds silly, but I think they are trying to put you into a corner. 600$ for such an image is just enough so you can pay it and it's too little for you to put an attorney onto. It's also not so little that you will stay away from paying until having like 3 reminders and they will give up the battle. It's exactly in the "right" zone...
I think they chose this amount just to pull some money. If I were you I would seriously ask for a detailed explanation of where the 600 dollars came from and also ask for the things you already mentioned yourself.
Anyway good luck with it.