$(function(){
    $("#globalnav a img").mouseover(function(){
        $(this).attr("src",$(this).attr("src").replace(/^(.+)(\.[a-z]+)$/, "$1_over$2"))
    }).mouseout(function(){
        $(this).attr("src",$(this).attr("src").replace(/^(.+)_over(\.[a-z]+)$/, "$1$2"));
    })
})