I think Custom Header Image is a cover art in WordPress blog.
The following photo is the origin of the custom header image in this blog.
It was taken in the pedestrian bridge near my house, the winter solstice, 2013.
In the left-hand side, we can see Tokyo Tower, Tokyo Sky Tree and the Venus.
In the center, Mt.Fuji can be.
Sorry, my poor English ability 
I haven’t thought about it like that, but you’re right: a custom header image can be cover art for a WordPress blog.
Thank you for your comment, very much.
My family and I had a great time visiting Tokyo Tower years ago. I do miss going outside and seeing Mt. Fuji. Thanks for the look back.
You’re welcome.
And, thank you for your comment.