By default, when posting using the RSS autoposter, Sendible takes the first suitable image it finds on the 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 this image instead 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: