<div><p>CSS</p>
<img src="http://www.w3tr.com/file/css.jpg" alt="..."/>
</div>

div {
  position relative;
  width: 100px;
  height: 100px;
}
div img {
  position: absolute;
  left:0;
  top:0;
  z-index: -1;
}
div p {
  font-size: 50px;
  color: red;
}