.element {
    width: 400px;
    height: 400px;
    border: solid red 1px;
    background: 
        url(../img/bgimage1.jpg) no-repeat top left,
        url(../img/bgimage-2.jpg) no-repeat top right,
        url(../img/bgimage-3.jpg) no-repeat bottom left,
        url(../img/bgimage-4.jpg) no-repeat bottom right;
}