[DanInMa]

ok well you cannot start and id with a number. your image alreayd has an id of bg, you want to change the id of bg?

<script>
$(function() {
var images = ["chococat.gif","comic.gif","devil.gif","eye.gif"];
$('<img>').attr({'src':''+images[1],'id':'bg','title':''}).appendTo('#bg-wrapper').parent().fadeIn(1000);
    $('.change').click(function(e) {
    e.preventDefault();
    var ranImgNum = Math.floor(Math.random()*images.length)
    var image = images[ranImg];
        $('#bg').parent().fadeOut(800, function() {
            $('#bg').attr({'src':image, 'id':'ranimageid'+ranImgNum}); 
              $(this).fadeIn(800);
        });
    });
});
</script>

if you want to change the id of bg, this will do it