Adding a css class to the linked images

Hi,
I want to add the fancybox effect to all images inside a post which are linking to the original image. For this purpose I need to add the css-class property to the surrounding <a>-Tag of the image. How can I do this?

So far I came to a result:

function addFancybox($content)
{
if (preg_match_all('/<a.*?><img.*?<\/a>/',$content,$matches))
{
print_r($matches);
}
}
add_filter( "the_content", "fancyfilter" );

But how to continue???

Thanx in advance!!!