Jacob G. Yeahhh so a little background here, embedded images within slack blocks use the Image block. There are 2 variants of this block: 1) URL or 2) Slack file-based. The second variant ties the message directly to the file object in slack... and that's where the privacy/access issues come in.
If you use the Block Kit Builder to paste a url from a Slack file, it will intelligently swap to the second variant. Since these Slack files are typically not accessible publicly, we can't display the Slack file-based variant of the Image block on the companion site. It is currently not supported.
The bottom line is, for Block Kit images you intend for the companion site, you should upload and host it at a publicly-accessible location (not Slack), and use that URL. I will also put a ticket on our backlog to explore support for the Slack file variant!
