Default ghost theme emits this error for google indexing if there is no image associated with a post. There are AMP versions of each ghost page generated that can be accessed by appending /amp in front of any post. The cursory research I undertook suggests that this is caused by the template ghost uses to generate the AMP format. I guess it doesn't handle the situation where there is no image by removing the unnecessary markup in AMP . Or perhaps an image is a requirement? I'm not sure at the moment.
You can determine the specific error by running the AMP version of your page through the Structured Data Testing Tool.
The quick way to fix this to add the post header image in Ghost.