Why not have one mosaic which embodies both ideas?
From the top left corner images must match up. From the bottom right corner, images don't need to match. Only allow people to pick a blank location that is along the next empty diagonal row, until it is complete. The mosaic can expand as the diagonals grow towards each other.
For the joined-up squares, people need to be able to reserve a spot and can't do their image until adjacent squares are complete. This caused some frustration with cre18 since people picked spots but then took a while to populate them. One of the big criticisms of cre18 was that as the number of pictures grew, the slower it got and viewing and zooming around the whole mosaic suffered.
Anyway, good luck. It's a simple idea but it's not so simple to have it work well.
Paul
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