The very first poster starts the story and the next poster continues the sentence with three works and the next person continues it and it just goes on and on.
Example.
1st Poster
Once Upon a time...
2nd Poster
Spammy in a
3rd poster
thong rode into
4th poster
a castle town
5th poster
and made love
6th poster
to Hooey and...
Something like that, be as creative as you can be, but add some humor to it, it can be quite fun, so...
Once upon a time...