From your screenshot, it looks like you are having the issue that is described in this post: Images not publishing to Discourse in WP 5.3 - #8 by simon. If that is the case, then I’m not sure there is a good solution other than disabling the Discourse download remote images to local site setting.
The other option is to not publish full post content to Discourse. If you publish excerpts instead of the full post, images should be correctly displayed when users click the “Show Full Post” button.