Find first image on paginated post for Pinterest

I’ve been looking for days, yet can’t find a good answer. Perhaps the SE crowd can be of assistance. Any help is much appreciated.

I am using a custom-coded pinterest button included on posts that pulls the first image of the post OR the featured image (if exists).

<span data-icon="0" aria-hidden="true"></span><a href="http://pinterest.com/pin/create/button/?url=<?php echo urlencode(get_permalink($post->ID)); ?>&media=<?php 
if ( (! has_post_thumbnail()) || ($paged != 0) ) { 
echo catch_that_image();
} else { 
// Has featured img
$image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'medium' );
echo $image[0]; 
} 
?>&description=<?php the_title(); ?>" class="pin-it-button" count-layout="horizontal">Pin It</a>

Now, the function that fetches the first image of a post looks as follows:

function catch_that_image() {
global $post, $posts;
$first_img = '';
ob_start();
ob_end_clean();
$output = preg_match_all('/<img.+src=['"]([^'"]+)['"].*>/i', $post->post_content, $matches);
$first_img = $matches [1] [0];

if(empty($first_img)){ //Defines a default image
$first_img = "http://static.purseblog.com/default.jpg";
}
return $first_img;
}

Any idea how to modify the catch_that_image function to take into account the pagination and find the first image on the current page a user browses? I’d like to then send the URL to pinterest for more accurate pinning.

Thanks so much in advance.