why does it matter so much anyway? wouldn't shape, color of the areola/nips and the size ratio of the areola to the tit be the aesthetics to judge by? any cup size you think it is just by seeing a bra under a shirt can be faked. Hell VS has a bra that let's you go up 2 cup sizes.
Yeah, it seems like anybody who put this much thought into it would realize that so many of the crucial measurements are impossible to judge from just a photograph.
Even if you snapped a picture of a girl's figure in profile -- how the fuck are you going to really judge her height? Nevermind that you can't tell if her bra is excessively padded or not. Nevermind that the possibility of fake breasts completely throws all these assumptions off, since they presume bodily proportions are the result of genetics (rather than storebought aftermarket boobs).
All the important measurements you are using for calculations are going to be guesses at best. I would say just go with your gut instinct, and realize that her breasts will probably be your best guess, plus-or-minus a cup size (probably minus).