Where to add css sprite code in wordpress theme?

Hi,

I’m trying to make some small changes to my website (www.etuibenau.com), but these small changes are becoming BIG problems. I’m trying to make the thumbnails on my ENTIRE site change from B&W in normal state to color when you hover. It seems my best option is css sprite, please let me know if you have a better solution.

So far I have this code

a{
display: block;
background: url(sprite.png) no-repeat;
height: 150px; width: 310px; }

a:hover{background-position: 0 -150px; }

I don’t where to paste it within ‘index.php’ seen below

<?php
$values = get_post_custom_values(“thumbnail”);
if (isset($values[0])) {
?>
” rel=”bookmark” title=”<?php the_title(); ?>”><img src=”<?php $values = get_post_custom_values(“thumbnail”); echo $values[0]; ?>” alt=”” />
<?php } ?>

Also what should ‘a’ and ‘sprite.png’ be in the following?

a{
display: block;
background: url(sprite.png) no-repeat;
height: 150px; width: 310px; }

a:hover{background-position: 0 -150px; }

I thought sprite.png might be something like ?php $values = get_post_custom_values(“thumbnail”)

Any help will be appreciated, thanks.