You are seeing Grey image on your post because you did not add featured image on your post. Henec by default theme has used grey image to make reminder that you forgot to add featured image on your post.
Every post should have its own one image to show at the front page. Hence it is required. But this is optional in the case of blog post section. In this section you can avoid grey image by unchecking UseBlankImage option.
- Go to admin menu >> appearance >> customizer >> theme option >> blog post section
- uncheck box UseBlankImage
But in all other section it is mandatory.