#imap a#seoul, #imap a#chungcheong, #imap a#kangwon, #imap a#jeolla {
display:block;
width:20px;
height:20px;
background:transparent url(./images/hover.png) -100px -100px no-repeat;
text-decoration:none;
z-index:20;
}
The hover.png is 20x20px. What is the effect of adding "-100px -100px" in the CSS ?