The posts were part of a surge of vitriol directed at Ms. Roper and her colleagues at Collective Shout, an Australian activist group, on X and other social media platforms. Some of it, including images of the women flayed, decapitated or fed into a wood chipper, was seemingly enabled — and given a visceral realism — by generative artificial intelligence. In some of the images, Ms. Roper was wearing a blue floral dress that she does, in fact, own.