http://lineage2.com/news/images/Piece1.jpg
This is just a theory:
I think this event works by wherever you find a piece of the puzzle you post it on the boards, for example, when you click piece one you get forwarded to the boards so you post the picture. Let's say we eventually find piece two, we would then post that on the forums as well, assuming that by clicking the piece two picture, we are redirected to the forums again.