So you can have an animated or interactive "wallpaper". The malicious wallpapers in the OP are hentai games.

The 'wallpapers' in question are pirated games made in renpy (python game engine) or rpgmaker (js based), which makes them a really good vector for malware. As another commenter noted this is a bizarrely common way for Chinese people to get porn through the great firewall.

Opensuse (pre 11 iirc) used to have a really cool background where the lighting changed throughout the day, that probably used an engine of some sort.

Because they are not static images. That's the whole gimmick

Because it's one of the only ways to get porn in China

Irrelevant comment, op said "this vulnerability isn't directly the result of using Steam", not that steam doesn't share responsibility

Why would seeing sexual imagery make you less rational? That doesn't make sense.

The study you mention say the people were already in an arousal state (that they had to induce themselves). It's very different from seeing images that you may simply ignore, evaluate differently, etc.

Also, there is the bias that if people are looking for such images (because they really want them), they are probably more willing to drop recommended practices, and hence make irrational moves. So irrationality doesn't come from seeing the images at the first place, but from their willingness to find / see such images.

>This is also why sexual imagery should generally be kept out of public spaces, not because of "puritanism" but because it just generally isn't a good idea to go around letting bad actors inhibiting people's executive function willy-nilly

Okay but presumeably humans adapt to the level of "sexuality" around them to some degree (like they do nearly every other stimulus), because otherwise you could show less prude cultures having lower ability to do "rational thought".

Nudity is normal all over the world and yet people seem to function just fine. What constitutes content that justifies sexual arousal is socially constructed!