The lingerie they're using is pretty much opaque tops and britches. In fact, Shannon wore those bikini briefs for her angel picture that IMHO were just as skimpy. I would say that if she does avoid this shoot, it should be less that it's the underwear than it is the fact they're posing in a Greek salad coated in oil - well this part looks like.
If it's the underwear, I would say Tyra does have precedence that she has shot in essentially underwear before - a Bikini - and as such, if she doesn't do the shoot she should be eliminated. If she does stay with no photo and someone else is eliminated who does the shot - unless absolutely horrible ALL shots, then we know who will be the winner of this season. Because save the very first cycle where TWO people didn't do the shoot, anyone who has refused to do a photoshoot has been eliminated: Anna in Cycle 2, and that one gal in Caridee's season who missed the shoot because she was "sick" but just didn't want to do the challenge it seemed.