You are seeing a Grey image on your post because you did not add a featured image on your post. Hence by default theme has used the grey image to make a reminder that you forgot to add the featured image on your post. Please see below image to know the location about setting feature image while creating post or editing post.
Every post should have its featured image to show them on the front page. Hence it is required. But this is optional in the case of the blog post section. In this section, you can avoid grey images by unchecking the UseBlankImage option.
- Go to admin menu >> appearance >> customizer >> theme option >> blog post section
- uncheck box UseBlankImage
But in all other section it is mandatory.