Actually I fail to see the problem with the current format. Maybe I don't get it, maybe it needs to be better explained.
There is a group dynamic that drives a bunch of motivated people to shoot more often, possibly to experiment, and to share their effort.
Not everyone is able to produce a top notch shot every day, and I certainly wouldn't if I tried. But to me the interest of this kind of experiment lies in the process itself, not in the daily outcome altough it is certainly necessary to post in order to keep going.
Is the problem related to quality or to server space?
If it is the former, it is fine to simply not follow the daily iterations and let the people motivated by the exercise move on. In that case why not let the flow come as it is, and ask people, either once a week or even less, to look back at the latest period and select one picture (possibly in a separate thread) and explain why? Wouldn't that exercise be interesting in itself? It may open the door to discussions among the group, and would certainly encourage non-participants (like me) to pay more attention.
If it is the latter, it needs to be clearly stated.
I don't believe that imposing a format will be fruitful if the participants are satisfied with the current frame.