to return to 1.5.1 you can download the files from the change log . You will need to use ftp and just overwrite the filles.
or the issue is that is not selecting the right size you can open the file include.php and modify this line return get_the_post_thumbnail( $post->ID, $imgsize ); for return get_the_post_thumbnail( $post->ID, $size );
This will be fix on the next release.