function initRollOverImages() {  
    var image_cache = new Object();  
    $(".btn img").not("[src*='_ov.']").each(function(i) {  
        var imgsrc = this.src;  
        var dot = this.src.lastIndexOf('.');  
        var imgsrc_on = this.src.substr(0, dot) + '_ov' + this.src.substr(dot, 4);  
        image_cache[this.src] = new Image();  
        image_cache[this.src].src = imgsrc_on;  
        $(this).hover(  
            function() { this.src = imgsrc_on; },  
            function() { this.src = imgsrc; }  
        );  
    });  
}  
$(document).ready(initRollOverImages);