We could agree on a number of thumbs to showcase (6, or 9, or whatever) so everyone does a number of those before him + 1 after. Excluding his own, which is a bit pointless. This way, even if someone forgets/refuses to put up the thumbnails, the chain remains intact using the links of others. It's a bit more difficult to do than the groups Yagho suggested, but since each one of us is going to do it once (+ one modification when the next image is added) I think it's within our capacities.