Thank you so much for using our theme SuperMag. Our theme is developed in such a way that you have to add featured image on every posts. If you missed to add the featured image on each post, in place of featured image, default no image will appear. You must need to add featured image.
If you are wondering how to add featured image, please read this post here.http://www.acmethemes.com/blog/2016/04/set-featured-image-on-post/
But in our premium version, you can hide/show featured image on single post and you can set your own custom images too.
If you have any query regarding our theme, please post your query here.