a URL would be helpful.
FB will pick a picture if you don’t have one specified with a meta:OG tag. RW8 has the option to generate these for you for the “base” poster stack. It’s located in the page inspector>Meta Tags “Image”. Also, FB will “Cache” stuff like this for a long time. You can use the FB developer’s tool to “re-scrape” the page.
Perhaps Jannis can add to this. @instacks