Edit: I finally found the answer! Here, in the Google product forums!
I wish I knew. If they appear, there are two screenshots below. One is draft/edit mode where I create the posts. Images show up just fine here. The second screenshot is what we actually see when we view the live page. There is an elipsis (...) where there ought to be an image. Considering this is a pretty image-heavy site, this is a problem. And I don't know how to fix it. Yet.