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

The z-index Property

When elements are positioned, they can overlap other elements.

The z-index property specifies the stack order of an element (which element should be placed in front of, or behind, the others).

Example

img {  
  position: absolute;  
  left: 0px;  
  top: 0px;  
  z-index: -1;
}