[Theme Feature – If you see this image, do not be alarmed. Contemplate the deeper meanings of the haiku. Then know that when McGurk can’t find an image for a post, it fills the empty space with this one.]
Most likely what happened is that there is no featured image and no secondary image specified for the post – like this one.
Another possibility is that the featured image you uploaded was not at least 800px x 800px. McGurk will try its best to create a square image from the featured image, but if the featured image is too small, well it just gives up.
Your best bet is to always upload two featured images. The main one can be any size ratio, but should be at least 800px each way. Then crop your main image square at least 800px both ways and upload that for your secondary.
You can skip the custom secondary image and McGurk will blindly center crop the main image, but nobody wants that, eh?