$(document).ready(function(){
    $("img.rollover").each(function() {
        var off = this.src;
        var dot = this.src.lastIndexOf('.');
        var on  = this.src.substr(0, dot) + '_on' + this.src.substr(dot, 4);
        if(off.indexOf("_on") == -1){
        	$(this).hover(
        		function() {this.src = on; },
        		function() { this.src = off;
        	});
        }
    });
});