All background utility classes have background-repeat: no-repeat.

Group of Classes	CSS
.img-background	background-size: cover; background-position: 50% 50%;
.img-background-100	background-size: 100%; background-position: top left;
.img-background-top	background-position: top center;

Group of Classes	CSS
.img-full-width	width: 100%; height: auto;
.img-full-size	width: 100%; height: 100%;

An element with position: absolute; will position itself within the context of it's closest position: relative; parent.

Group of Classes	CSS
.center	display: inline-block; left: 50%; position: absolute; top: 50%; transform: translate(-50%, -50%); vertical-align: middle;
.absolute-cover	position: absolute; top: 0; bottom: 0; left: 0; right: 0;

Class	CSS
.img-border	border-color: rgba($cr-gray-lighter, .4); border-style: solid; border-width: 2px;

To make an image circular, use the class .img-circle which is provided by Bootstrap here.