The way it works is that the HTML preview in the post retains fidelity with the original content. You’d need a custom plugin to do this.
The basic problem is the same. Trying to get a Wordpress post that is almost entirely images to be 1:1 between Wordpress and Discourse is not what the software is designed for. The reason topic embeds (what powers this on the Discourse side) uses the Readability gem is that the assumption is that you’re dealing with content that is read (i.e. text).
It would be possible to have a reliable photo gallery sync between Wordpress and Discourse, however you’d need to design the customisations (Wordpress plugin and Discourse plugin) to work that way.