Setting a background
To add a background insert the following code at the top of your:<body style="background-image:url(URL);background-size:cover;">In the code above set the URL to the background you want.
Things that won't work
If you don't put the URL inside url(URL):<body style="background-image:http://throw-homepage.github.io/img/strahov-abbey-library.jpg;background-size:cover;">