Display only thumbnail (Custom thumbnail size)

Hi, i was looking for a code to do that and i found this one.

<?php
$footer_thumbs = get_posts('cat=9&numberposts=9&orderby=rand');
if( $footer_thumbs ) :
  foreach( $footer_thumbs as $footer_thumb ) {
  setup_postdata($footer_thumb);
echo '<a> ID) . '">' . get_the_post_thumbnail($footer_thumb->ID) . '</a>';
}
endif;
?>

[please use the code button to mark any code to keep it readable – forum guidelines – the code above is partly broken]

So, this one works but:

1.- Is it correct, or are there better options?
2.- I want to use a custom size thumbnail, but haven’t found how to.

Any help will be great.

Thank