Well it's not a secret, but it's not obvious for most teenagers. And may be not either for people without high degrees education.
A lot of people, and I suppose not only in France, are not at peace with their body shape, even if they're not obese nor anorexic. Been exposed to not really existing bodies images can be painful for self esteem.
Of course, computer modified photos are recent, but fashion already used lightning and tricking exposure when producing photos from film since, well, the fashion press exists I guess.