This is simple. You'll rape your computer's CPU and GPU by playing this game, but it's simple and worth it.
Step 1: One player posts an image.
Step 2: Another player posts an edited version of the image above. It doesn't matter if you just overlay something, or you're an avid tenniser who applies filters to anything. Basically, you can do anything with the image, just post the end result in the thread.
Step 3: Repeat step 2 infinitely. If someone is ninja'd, continue with the post that ninja'd the below post.
Good luck; and anyone can start!
