Solution: It is necessary to activate the Open Graph options in the Page settings.
Click the cog icon by the Blog Posts page. Select your image from the Open Graph section. I chose Main Image to be my default image. Then re-index your search, via the big cog icon on the far left (under all the other layout options). The republish your site.
Make sure you have an image in your search results layout, and in Image Settings that the tick box is ticked to get the image from Search Index.