div.circle-avatar{
/* make it responsive */
max-width: 100%;
width:100%;
height:auto;
display:block;

/* make it a circle */
border-radius:50%;

/* Centering on image`s center*/
background-position-y: center;
background-position-x: center;
background-repeat: no-repeat;

/* it makes the clue thing, takes smaller dimension to fill div */
background-size: cover;

/* it is optional, for making this div centered in parent*/
margin: 0 auto;
top: 0;
left: 0;
right: 0;
bottom: 0;
}