[xelawho]

in that case it's simple - just replace the tn_ and use that for the src...

$('.thumbs').click(function(e) {
    e.preventDefault();
    var image = $(this).attr('src').replace("tn_","")
       $('#bg').parent().fadeOut(800, function() {
            $('#bg').attr('src',image); 
              $(this).fadeIn(800);
        }); 
    });