Not without hacking the code.
It isn't that hard to do though.
Open up rsgallery2.php and find around line 285 the following lines:
<?php
HTML_RSGALLERY::RSGalleryList($rows, NULL, $catid ); //This show the subcats if they are available
HTML_RSGALLERY::RSShowPictures ($catid, $limit, $limitstart); //Show the pictures
?>
Just switch the two rows and it will show thumbs first and subcats below that.
Good luck,