Thank you for using our theme SuperMag. In case of this thumbnail problem, you have to add featured image on every post to remove this default image. If you missed to add feature image on each post default image will appear on post. To get rid of this, you have to add featured image on each post to replace the default image. Or, if you want to remove the featured image completely, you have to customize the theme or go to the premium version of it. We developed the theme on the way that, each post must contain featured image. Please look here to find the difference.
If you are confuse how to add feature image on post, please have a look here. http://www.acmethemes.com/blog/2016/04/set-featured-image-on-post/