It sounds like you are running into a similar issue to what id described here: Fix broken images for posts created by the WP Discourse and RSS plugins.
You can confirm that by opening the post in the editor and having a look at the markdown upload link. If the upload link is surrounded by HTML tags, try editing the post to add an empty line between the opening HTML tag and the upload link.
As an example, the following markdown will create a broken image:
<p></p>
If this is what you’re seeing, manually editing the post to something like this should fix the image:
<p>
</p>