Only posts with images/thumbnails

You can’t do that with this plugin directly.

You might need to try this using an independent set of code using the plugin functions
https://github.com/WebberZone/code-library/blob/master/contextual-related-posts/crp-api-example.php

in combination with meta queries – https://wordpress.stackexchange.com/questions/89202/query-posts-only-with-featured-image

Thank you! This also helped me with another minor question.

Thanks for confirming – would love to see your implementation of this