function ImgMouseOver(imgId) {
    var img = document.getElementById(imgId);
    var imgSrc = img.src;
    var ext = imgSrc.substring(imgSrc.lastIndexOf(".") + 1);
    var overSrc = imgSrc.substring(0, imgSrc.length - ext.length - 1) + "_over." + ext;
    img.onmouseover = function() {
        this.src = overSrc;
    };
    img.onmouseout = function() {
        this.src = imgSrc;
    };
}