A-ha, I’ve seen this a few times with Thunderbird (but never with images, just with text so it didn’t really impact the usability/readability of the email).
I imagine what’s going on here is that somewhere in the HTML → Markdown conversion some whitespace is being added (or not removed) producing something like this:
<img blah blah blah blah>
which is rendered like so:
<img blah blah blah blah>
Getting the raw markdown output of the post will confirm this. Do that by replacing the /t/<slug>/ part of the url with /raw/, eg:
https://meta.discourse.org/t/inline-image-post-via-thunderbird-not-processed-properly/94961/5 → https://meta.discourse.org/raw/94961/5