$(function() {
    $('.gallary-thumbnail').each(function() {
        var url = $(this).attr('src').replace(/\.[a-z]+$/, "-big$&");
        (new Image).src = url;
	$(this).hover(function() {
            $(this).closest('.gallary').find('.gallary-big')
                   .attr('src', url)
        });
    });
});