img {
position : absolute;
top: -moz-calc(100% - 50% - (305px / 2));
left: -moz-calc(100% - 50% - (320px / 2));
top: -webkit-calc(100% - 50% - (305px / 2));
left: -webkit-calc(100% - 50% - (320px / 2));
top: calc(100% - 50% - (305px / 2));
left: calc(100% - 50% - (320px / 2));
}
http://www.suburban-glory.com/blog.php?page=155