Add background-position:center;
in #cross:before, #cross:after
#cross {
width: 100px;
height: 100px;
position: relative;
}
#cross:before, #cross:after {
content: "";
position: absolute;
z-index: -1;
background-size:100px 100px;
background-image:url('http://www.gm-consult.de/uploads/pics/homer-simpson_20.jpg');
background-position:center;
}
#cross:before {
left: 50%;
width: 30%;
margin-left: -15%;
height: 100%;
-webkit-transform: rotate(45deg);
-moz-transform: rotate(45deg);
}
#cross:after {
top: 50%;
height: 30%;
margin-top: -15%;
width: 100%;
-webkit-transform: rotate(45deg);
-moz-transform: rotate(45deg);
}