z index

img {
  position: absolute; 
  /*position: relative;
  //position: fixed;*/
  left: 0px;
  top: 0px;
  z-index: -1;
}

The z-index property specifies the stack order of an element.

An element with greater stack order is always in front of an element with a lower stack order.

Note: z-index only works on positioned elements (position: absolute, position: relative, position: fixed, or position: sticky).

#nav {
  position: fixed;
  ...
  z-index: 2;
}
#main {
  position: absolute;
  ...
  z-index: 1;
}

Definition and Usage. The z-index property specifies the stack order of an element. An element with greater stack order is always in front of an element with a lower stack order. Note: z-index only works on positioned elements (position: absolute, position: relative, position: fixed, or position: sticky).
Default value: auto

  
    img
 {

     
 position: absolute;

     
 left: 0px;

     
 top: 0px;

     
 z-index: -1;

 }
  
 

z-index