Hello,
this is somewhat tricky to debug because “view online” pages of newsletters are not real Wp pages, as they are dynamically created. So there’s no way to customize open graph settings for those pages. The Facebook scraper could take the first image on the newsletter, for example, or the general website image, if that’s set by a Seo plugin. You can verify the behaviour by googling Open graph debugger.
A workaround for this might be setting a general website opengraph cover image, optimized, which will be detected by Facebook (and most other social networks).
Michael