Have a picture on the home/landing page! Show what kind of cool stuff can be made by this. Maybe an awesome, mainly complete picture that still has some "puzzle pieces" blank on it. Then the call to action text next to it is "Help complete this picture" or something. Basically you want to make it clear that by uploading a single small piece you are contributing to a larger piece of art.
The feedback upon "clicking to reserve a tile" is not good. I kept double clicking because nothing was happening. It took me a long time to realize that the details were being populated over in the left column. A better design would be a lightbox that expands the single tile to full screen when you click a tile. Enforces the fact that you're now editing THIS tile. This would be an especially great interaction if you included a rudimentary web drawing tool for directly editing the new big tile.