I feel we should not set a goal of making a calendar that is "fair". We should be looking for photos that are artistic and "good to look at". After all, we will be looking at these pictures for a whole month at a time.
I would be more in favor of photos showing the planes flying or "action" shots where we really showcase the different ways people use their planes. I really wouldn't want to see a bunch of "hero" shots of planes alone on the ramp.
The only rule I can see putting in place is that each plane is only in one picture, so that we get as many different airplanes in as we can.
I foresee a basic process of people nominating photos, an editor (or group of editors) review the nominated pictures and make sure they will all work in a calendar format (i.e. cropping the picture to fit in the aspect ratio of the calendar may ruin it), all approved pictures are put up for a vote, and whichever ones get the most votes end up in the calendar. Some months may have multiple pictures on them, and others may have only one picture.
Dates on the calendar can show the dates of the big fly-ins (Sun-N-Fun, Oshkosh, Clinton), as well as local events and club-only events.