So, so sorry you had to see that, and thank you for protecting the rest of us from seeing it.

On traditional forums, you'd have a lot of control over the posting of images.

If you don't wish to block images entirely, you could block new members from uploading images, or even from sharing links. You could set things up so they'd have to earn the right to post by being active for a randomised amount of time, and have made a randomised number of posts/comments. You could add manual review to that, so that once a member has ostensibly been around long enough and participated enough, admin look at their activity pattern as well as their words to assess if they should be taken off probation or not... Members who have been inactive for a while could have image posting abilities revoked and be put through a similar probation if they return. You could totally block all members from sharing images & links via DM, and admin email accounts could be set to reject images.

It is probably possible to obtain the means to reject images which could contain any sexual content (checked against a database of sexual material which does not involve minors), and you could probably also reject images which could contain children and which might not be wholesome (checked against a database of normal images of children).

Aside from the topic in hand, a forum might decide to block all images of children, because children aren't really in a position to consent to their images being shared online. That gets tricky when it comes to late teens & early 20s, but if you've successfully filtered out infants, young children, pre-teens & early teens as well as all sexual content, it is very unlikely that images of teenagers being abused would get through.

Insisting that images are not uploaded directly, but via links to image hosting sites, might give admin an extra layer of protection, as the hosting sites have their own anti-CSAM mechanisms. You'd probably want to whitelist permitted sites. You might also want a slight delay between the posting of an image link and the image appearing on Beehaw - this would allow time for the image hosting site to find & remove any problem images before they could appear on Beehaw (though I'd imagine these things are pretty damn fast by now).

You could also insist that members who wish to post images or links to images can only do so if they have their VPN and other privacy preserving methods disabled. Most members wouldn't be super-enthused about this, until they've developed trust in the admin of the site, but anyone hoping to share images of children being abused or other illegal content will just go elsewhere.

Admin would probably need to be able to receive images of screenshots from members trying to report technical issues, but those should be relatively easy to whitelist with a bot of some sort? Or maybe there's some nifty plugin for this?

Really though, blocking all images is going to be your best bet. I like the idea of just having the Beehaw bee drawings. You could possibly let us have access to a selection of avatars to pick, or have a little draw plugin so members can draw their own. On that note, those collaborative drawing plugin things can be a fun addition to a site... If someone is very keen for others to see a particular image, they can explain how to find it, or they can organise to connect with each other off Beehaw.