try something like this before using javascript!
.center {
width: 300px;
height: 300px;
position: absolute;
left: 50%;
top: 50%;
margin-left: -150px;
margin-top: -150px;
}
or this!
.image_container {
width: 300px;
height: 300px;
background: #eee;
text-align: center;
line-height: 300px;
}
.image_container img {
vertical-align: middle;
}