The gimmick here is that it's a "naked calendar", i.e. the people involved aren't wearing any clothes so they're relying on strategically placed objects/limbs to preserve their modesty. These have been around for a few years, and typically you don't see anything that you wouldn't see at the beach, with the possible exception of bare bums. I think that some of these calendars work better than others, and there are a couple of simple guidelines to follow:
1. If you had the exact same picture, but everyone was wearing clothes, you wouldn't notice anything unusual about it. For instance, several famous cyclists have posed naked on bikes (Lance Armstrong, Victoria Pendleton, and Rebecca Romero). Those weren't for calendars, but they illustrate the point.
2. People shouldn't wear underwear, or at least it shouldn't be visible. For instance, if someone's carrying a surfboard and I can't see what's on the other side, they can wear whatever they like. Other clothing is fine, particularly if it's unusual, e.g. a woolly hat or a pair of wellies.
The common theme is that the people involved shouldn't appear to be self-conscious or embarrassed by what they're doing. The illusion is that they're just acting normally and it's sheer chance (angle/timing) that the picture didn't reveal anything. I know that there are lots of people who would hate the idea of taking part in a calendar like this, and that's absolutely fine - I'm certainly not trying to pressure anyone into doing something that they're not comfortable with. However, when I see a picture of someone who is using their hands to cover themself then I just think "if it bothers you that much then put your clothes back on!"
Looking at this calendar in particular, it's fine on point 2, and pretty good on point 1. I suspect that they've used Photoshop in a few places (e.g. to position flying objects in the appropriate place), which is cheating a bit, but I can live with that. Ultimately, though, the purpose of the calendar is to raise money/awareness, so I'm happy with my purchase, and I recommend it to anyone else who's still looking for a calendar for next year.
