it is working great now, see it here in live action:
http://blog.uhlig.at/index.php?x=mosaic
only thing is funny: in InternetExplorer (6 and 7) the thumbs are created row for row and all are shown.
Firefox first creates the whole div and then fills it with thumbs in random order. Doing this it "forgets" some of them so there are "wholes " between, it feels like this is concerning always the same images