Is working on search engine optimization for websites that display pictures of women's underwear on models' bodies considered forbidden, even if the products themselves are permissible, given the assistance in spreading the viewing of these images?

It is not unlawful to work on improving websites that offer permissible things, even if they contain impermissible images, because these images are not the intended subject of promotion. As for improving impermissible images and helping them appear in search results, that is not permissible.