What this woman is doing is entirely within traditional norms for women and is in no way shape or form challenging conventional ideas. Quite the opposite.
It was always acceptable for women to look attractive as long as they didn't cross the line marked "slutty", yes - which I'd argue that in some ways, this picture does. Certainly some previous reactions from posters have suggested so.
But more importantly, women flaunting themselves in the past very rarely were doing this entirely for the reason that they liked the way they looked. Or at least, they weren't perceived as doing so.
Women are posting pictures of themselves online. A lot of the time they're not making money, these are self-taken pictures, they're in a bathroom, they're alone. These pictures are often taken for the sole reason of "I look good" and there's an increasingly outspoken number of women openly rejecting the concept that these pictures are taken for men's appreciation.
Have you seen the experiments where the women agree with compliments they're given online? The reactions are incredible. These women are not "modest", that much is clear. The women look good, and many of them are posting them simply because they like looking good and other people seeing so too. In older times, women who had similar pictures distributed in a public fashion were making money from it - an acceptable, though stigmatised reason, and it was often perceived as for the attentions of men.
They also weren't taken by the women themselves. Lots of women didn't really have a choice in the matter in a commercial context (and honestly I think modelling, films etc. is the only thing comparable to social media in this sense.) So women weren't often deciding which picture looked good or even to take them.
But as you can see from this thread, there's an increasing awareness that women are taking pictures simply because they like their bodies and want to post the photograph so other people can see. Which doesn't digest well.