Change the RSS auto-post default image
By default, when posting using the RSS auto-poster, Sendible takes the first suitable image it finds on the web page. However, there are WordPress plugins you can install to make sure that the
og:image headers are included in your posts. This way Sendible will use the chosen image when posting the link to Facebook.
Here is an example of a WordPress plugin you could use:
This article explains more about the Facebook Open Graph meta tags that you should include with each post: