Well, those may not be coming from the actual gallery, the imge URLs are different in the page source, and in the search.
Can you check if on any of those posts there is a featured image set perhaps? Maybe that is the one, the plugin actually fetched.
If the option I suggested did not work, then then this may not be possible. I suspect the gallery may not be rendered only on an actual pageview, so the plugin can’t “see” the images in the page source.
I can double check the configuration if you want to, however I could not find a way to log-in. Is there a URL I need to visit for that?
The custom field you tried there will not work that way. The field has to be an image field, or a text field containing the image. I guess the gallery is some sort of a section/object or a complex repeater field of some sort.