Above css, like any other should be pasted under Theme options > Custom CSS & JS > Custom CSS section. It works on 100% because we tested it. However, if it does not work for you, it means you did a mistake in your custom css section and that's why any css below that mistake, does not work. Move the css we gave you on 1st position then and you will see the difference.
Yes, that is where I added the CSS. I moved it to the 1st position, and there is no difference. I cannot see the header background image for single posts.
Please see the image and let me know if I'm doing anything wrong.
Comments
if you want to set the same header image for single posts, just use "Header Image" option which is available for each post.
This code don't work because of your comments, please remove them or use css not html comments
http://www.w3schools.com/css/css_syntax.asp
Thanks!