var sign = 0;

function rollover (imgname) {
if (sign != imgname){

var path = document.images[imgname].src.split('.gif');
var act = path[0] + '_high.gif';

document.images[imgname].src = act;
sign = imgname;
}
}

function rollout (imgname) {
if (sign = imgname) {
var path = document.images[imgname].src.split('_high');
var inact = path[0] + path[1];
document.images[imgname].src = inact;
sign = 0; 
}
}