For an exchange such as this, this is how I personally see the options:
* Stay with the current game plan - Have a period of time to nominate to be included in the exchange, have a set time to print and send to the co-ordinator and when all the sets are ready to go, distribute.
* Modify the current game plan to ensure that prints are sent out to the final destinations by a set date, regardless of all the prints being available - OK, lets say we have 20 nominate, but 18 sets of prints are received at the cut off date. Since we would break that up into two groups of 10, the way I see it is that each person would get 9 new prints set back to them, plus one of their own prints sent back. Those who didn't get their prints in by the cut off date simply miss out. If they wish to participate again in the future, there would have to be some makeup or penalty (I.E., miss out on a round or ensure that they do a bit of a double up to make up for the prints they didn't send). The downside with this is if there is a big number of drop outs, you could get a big number of you own prints sent back
* Adopt Seabirds idea (not too bad, just a bit of a pain for the person organising it). At least you know that if you send out 5 prints, you will get 5 different prints back….
Any other thoughts?