In the free theme, you can only choose the Pages to display in the Featured Area. If you don’t set a featured image for the page you have selected to display in Featured Area, it will show grey background. So, we suggest you add the featured image for all your posts and featured area pages.
In order to set the featured image for that post, go to Admin Dashboard > Posts > All Posts > Edit the post > Set the featured image. Check the screenshot – http://prntscr.com/ko2whd
In all our Pro themes, you can set a custom Featured Area content.