set img as background

I would like to know how I can set my background as an image using

<img src="" alt="sample" width="480" height="500" border="0"> 

I know I don't have an image name that is because I'm picking an image from a dropdown menu. My problem is how do I integrate the code about to be displayed as a background image.

This is what I need to integrate it to.

.container {
  background-color: lightgrey;
  width: 500px;
  height: 500px;
  border: 2px solid;
  position: relative;
  overflow: auto;
}