Rules are as follows. One person starts with the scene. The first person posting sets up a small scene in the story. Like you enter a tavern, and see two half-orcs wrestling in mud. The first person also sets up the gaming system that the scene takes place in. Like D&D, Warhammer, Star Trek, Deadlands, GURPS, etc.
The scene you set can't be longer then a paragraph. The next person rolls a d6 to elaborate on the story. The catch is you can never post more then a paragraph. You can include any and all pictures you want. But the D6 is what determines what type of RP element you enter into the scene and how it progresses.
Please remember to keep everything PG-13. A paragraph is the limit, but 1-2 sentences is the preferred style and will work just fine.