How to do rollover buttons like this

i was wondering how to make rollover buttons like this ones navigation bar

http://www.project-ecko.co.cc/

Go to your css style sheet and put in this code.

a.mainmenu img {
-khtml-opacity : 0.8;
-moz-opacity : 0.8;
filter : alpha(opacity=80);
opacity : 0.8;
}
a.mainmenu:hover img {
-khtml-opacity : 1;
-moz-opacity : 1;
filter : alpha(opacity=100);
opacity : 1;
}

Edit...that's providing you already have navigation buttons

thank you so much