Ah, that’s because of deficiencies in the Wordpress-to-Discourse plugin. Specifically, it posts rendered HTML as the message. This works, because Markdown allows pretty-much arbitrary HTML as well.[1]
By default, that plugin uploads images and inserts image markdown, but there is a bug[2] where the markdown and html were running together in an invalid way, makingimages were coming out broken. The workaround is to tell the plugin to just leave HTML image tags pointing to the original – and those don’t have the nice scaling feature.
I’m going to go back to upstream discourse and re-discuss the problem and see if I can get some movement.