how to use a pic for background?

Where is the page?

For plain HTML you can do that following:

<img src = "location"></img>

You can also you CSS.

#banner
{
background: #5577FF url(/images/test.jpg) no-repeat;
height: 90px;
width: 540px;
}

with your HTML looking like:

<div id="banner"></div>

I’m trying to use a pic for the entire background of my blog

https://tamba2.org.uk/wordpress/graphicalcss/rap/index.html