Sorry for the delay, I just tried this and it does not work, unfortunately.
Also, I just published a new post which has centered square images, on discourse it is stretching the images to fill the container even though the html looks correct.
Original post: Finding Your Muse - Nature Photographers Network
I have the articles section on discourse locked down to members, so here is a screenshot and a portion of the html:
<small>Originally published at: https://naturephotographers.network/articles/finding-your-muse/
</small><br>
<div class="wp-block-image"><figure class="aligncenter is-resized"><img src="https://npn.sfo2.cdn.digitaloceanspaces.com/wp-content/uploads/20190115215008/Music-From-Dark-Spaces-IX-1024x1024.jpg" alt="Music From Dark Spaces IX" class="wp-image-3589" width="512" height="512" srcset="https://npn.sfo2.cdn.digitaloceanspaces.com/wp-content/uploads/20190115215008/Music-From-Dark-Spaces-IX-1024x1024.jpg 1024w, https://npn.sfo2.cdn.digitaloceanspaces.com/wp-content/uploads/20190115215008/Music-From-Dark-Spaces-IX-150x150.jpg 150w, https://npn.sfo2.cdn.digitaloceanspaces.com/wp-content/uploads/20190115215008/Music-From-Dark-Spaces-IX-300x300.jpg 300w, https://npn.sfo2.cdn.digitaloceanspaces.com/wp-content/uploads/20190115215008/Music-From-Dark-Spaces-IX-768x768.jpg 768w, https://npn.sfo2.cdn.digitaloceanspaces.com/wp-content/uploads/20190115215008/Music-From-Dark-Spaces-IX-600x600.jpg 600w, https://npn.sfo2.cdn.digitaloceanspaces.com/wp-content/uploads/20190115215008/Music-From-Dark-Spaces-IX-100x100.jpg 100w, https://npn.sfo2.cdn.digitaloceanspaces.com/wp-content/uploads/20190115215008/Music-From-Dark-Spaces-IX.jpg 1500w" sizes="(max-width: 512px) 100vw, 512px" /></figure></div>