$(document).ready(function(){
    $("img.imgover").each(function() {
        var off = this.src;
        var dot = this.src.lastIndexOf('.');
        var on = this.src.substr(0, dot) +
            '_o' + this.src.substr(dot, 4);
        $(this).hover(
            function() { this.src = on; },
            function() { this.src = off; });
    });
});