This is all because the developers in Nextcloud are "driven" about security. Maybe it's right.
In general, the problem is that if you give the user the opportunity to upload a photo, and then use a DIRECT link to this file, then this user can somehow hack something in this way. Therefore - for no reason
BUT. If you GENERATE another preview image from the original picture, then you can.
To do this, add /preview at the end of the URL. Like this:
https://cloud.ustim.ru/s/gdMBq4mEx63qpSz/preview
And such a link seems to work on forums and external sites.
Another option is to make another Apache (or Nginx) vhost with Directory Root pointing to your nextcloud/data. BUT here you need to understand what you are doing, because in this scenario, files that you might not want to make public will become public ...