.covered {
    position:relative; /* make a new "render context", so absolute positioning is relative to this parent container */

    padding:30px; /* only needed for this demo */
}

.covered-img {
    background: url('../img/img10.png');
    opacity: .25;

    background-size:cover; /* cover will scale the image so that the smallest dimension = the widest dimension of the box */
    background-position:center; /* vs the top-left that is default */

    position:absolute; /* take me out of the render context! let me define my own positioning */
    top:0;bottom:0;left:0;right:0; /* this could also work with width:100%; height:100%;, but is simpler */
}