$(function() {
    var images = $('.nav-but1 img');
    images.each(function () {
        var that = $(this);
        var filename = that.attr('src')
        that.data('originalimg', filename);
        that.data('hoverimg', filename.replace('01', '02'));
    });
    
    images.hover(function () {
            var that = $(this);
            that.attr('src',  that.data('hoverimg'));
        },
        function () {
            var that = $(this);
            that.attr('src',  that.data('originalimg'));
        });
});