You could use getRandomImagesAlbum() to get an image for your thumb. Then you would have to use
` print "<img src='".
$randomImage->getCustomImage(85, null, null, null, null, null, null).
"' width=620 height=180 alt=".'"'.
htmlspecialchars($randomAlt1, ENT_QUOTES).
":n".htmlspecialchars($randomImage->getTitle(), ENT_QUOTES).
'" />';`
Caveat: I have not tried this!