Hey ADAMWebDesign,

I don't know anything about JS menus, but I use this CSS background postion swap all the time for links. It might help.

Put all your button images on one picture, aligned vertically with a small space between. Then use CSS to offset the picture.

#menu a {background: url("images/button.gif") no-repeat 0% 0%;
background-position: 0 0px;
height: 18px;
width: 110px;
display: block;
font: bold 10px/1 Tahoma;
color: #FFFFFF;
text-decoration: none;
text-align: center;
margin:0px;
padding:0px;
}
#menu a:hover {background-position: 0 -21px;
color: #00FF66;
height: 18px;
width: 110px;
padding:0px;
text-decoration: none;
}
#menu a:active {color: #F0FFF0;
text-decoration: none;
background: url("images/button.gif") no-repeat 0 -21px;
}


This way the image loads all your button pictures at once.

Might help.
Drummin