If you are using Nexus WordPress Theme from Elegant Themes, you’ll notice that the header image on top of posts doesn’t look very good. It simply displays the overblown or stretched version of the ‘featured image’ you have set for the post.
This is because you need to specify a separate header image for posts. If you don’t specify it, the default ‘featured image’ is used instead of it which doesn’t look good.
To use a different header image for single posts, scroll down a bit while writing the post to find the ‘ET Settings’ box and upload header image in it.
Once its done, the post will use a separate header image specified by you.