There are. They are easy to abuse too, and cause a lot of other issues, along with not being 100% efficient. Apple tried that and was met with appropriate backlash. There are many issues : training data is one, but also it is impossible to rule out false positive/false negative automatically, and it is relatively easy (for now) to doctor pictures to pass through. A nefarious actor could easily bypass these. There’s also the case of false positive that can very quickly push some bystander under the bus, and knowing how moderate and understanding internet is… yup.
It is also risky, as this will effectively be a censorship tool; they are often setup in the pretense of “helping”, but once they’re up, it all depends on who steers it. Such responsibility can hardly fall on moderators/admin of lemmy, and it would also be problematic to handle them on a more nationwide (or more) level, since it would give incredible censorship power to authorities.
And a bigger bottom-line is that working hard to prevent these kind of content from reaching us, while it have an obvious upside, also does nothing in respect to the actual issue of the content existing and being created.
tl;dr: there is no easy solution that doesn’t come with one hell of a string attached to it.
On the other hand, it is quite hard to hide yourself from authorities online, and this kind of behavior (I hope, somehow, that the people that posted these only do so to be toxic to lemmy and not to actually disseminate content) should lead to some action from authorities, getting to actual people and subsequently moving upward the stream to actually act on it. Hopefully.