Agreed, it's 2019 I too would rather have the computer vision community use a different image instead Lenna, perhaps the proposed Fabio Lanzoni[1] or the Kodak image suite[2].
I like the idea of changing it, but neither of those options seem good. Fabio is just a response to Lenna - it seems like we can do better. And those Kodak images are really surprisingly bad. Consistently drab colors, blown highlights, and really badly composed pictures of dated subjects. Is there something I'm missing that makes those images important?
Fabio is fine, and he is proof that there's nothing wrong with Lenna either. Papers with algorithms relevant to photographs of humans should probably include both images, as well as a few more to include some racial diversity.
Images of humans are important to humans. If dogs were scientists, they'd probably devote lots of study to the scents of other dogs.
The image may be fine if we strip it of the context, but the world doesn't work like that. The image is famously known to be cut out from a larger nude photo, which was chosen for its sexual appeal, and published in a magazine of sexual content. That is not an appropriate choice for most academic publications.
If you want to use it to demo your nudity detection algorithm, feel free.
A noted judicial maxim, "I know it when I see it", has been criticized as trivial. Here you prove that not to be the case. You have a serious disagreement with the maxim, in that the purely visual properties of this image are not what make it objectionable to you. Instead you dislike the history of the image. But its nearly five decades of use in this field are the historical reason it is used today. You're advocating for your historical judgment over the different historical judgment of scholars in the field.
That could be reasonable. For instance, the fact that statues commemorating the Confederacy have some historical relevance doesn't make up for the fact that they mostly celebrate the awful institution of slavery and a society that was built upon that. We therefore disregard the "historians" who advocate for their continued intrusion into our public spaces.
However, those statues are quite explicit in their celebration of that odious time. This image is simply a beautiful image of a beautiful human. And its history is far from odious. Even the larger image is hardly prurient: secondary sexual characteristics are viewed obliquely. Most women have no problem with that. (Instead they object to the continued vilification of breasts in the form of breastfeeding bans etc., which vilification you continue here.) Lenna herself certainly has no problem with the image: she has publicly stated she values her appearance in the best-selling Playboy of all time, and its subsequent second life in academia. There is nothing wrong with an image of an unclothed human body, and this image is tamer than many such images.
Few women appreciate your advocacy here. Unlike you they don't assume that everyone is either a Puritan or a junior high schoolboy. I can think of few things less interesting or important than a "nudity detection algorithm".
It's not clear why the objections lodged against the original image could not also be lodged against Kodak's "kodim04" image. Both are images generated in the 1970s that portray smiling attractive young women of Scandinavian descent with bare shoulders.
[1] https://www.cmc.edu/news/every-picture-tells-a-story
[2] http://r0k.us/graphics/kodak/