Thanks for your comment, but I don't follow completely.
The Board generates pages, embedding text, pictures, links to attachment downloads etc. inside HTML, right?
Why could there not be two separate privs, one of which says "OK, user is allowed to see pictures, build the page with pictures in it", and the other of which says "OK, user is allowed to see availability of and use links to download attached files, so build the page with those links in it."
IOW, no priv for pics, no pics in the page, no priv for downloads, no file attachment download links in the page.
PhpBB3 controls the privs database, the content database, the files database, and the page generation - I don't track with how "it can't be done" can be the bedrock final answer here.