No worries. I think you are alright to borrow an image for a PC token in an rpg game.
Yes you are all still mounted. But this round and this round only I'll let everyone have a free dismount and a surprise action otherwise your only action will be to dismount ... which would be kind of crappy ...
These are riding horses and no one has the mounted combat feat so fighting on back of the horses wouldn't be a great idea. Without bothering to look up a rule, I'd just apply a DMs fiat -2 to attack rolls made by anyone who stays mounted.
I guess taking the horses into combat would be feasible, and they would move faster than you could walking, but they are going to freak out pretty fast with the chaos that's going on in the plaza ... but an option is an option, and with good charisma skills or nature skill a non trained rider could keep control of them. Horses lets say they can move 8 squares at normal pace.