Stretch background image in CSS

[jcdesigns]

How can I automatically stretch a sidebar background image to the bottom in css? Here is an example in w3schools but I could not get it to work on the site I am working on. Please advise. Thanks

https://www.w3schools.com/cssref/try...ackground-size

border: 2px solid black;
padding: 25px;
background: url(mountain.jpg);
background-repeat: no-repeat repeat;

[molendijk]

Left:

position: absolute;
left:0; 
top:0; 
bottom:0; 
width: 200px;
border: 2px solid black;
padding: 25px;
background: url(mountain.jpg);
background-repeat: no-repeat;
background-size: cover;

Right:

position: absolute;
top:0; 
right:0; 
bottom:0; 
width: 200px;
border: 2px solid black;
padding: 25px;
background: url(mountain.jpg);
background-repeat: no-repeat;
background-size: cover;