Finds the image id given the source. Or returns false.

function get_attachment_id_from_src( $attachment_url = '' ) { 
	global $wpdb;
	$attachment_id = false;
 
	// If there is no url, return.
	if ( '' == $attachment_url )
		return $attachment_id;
 	
	$query = "SELECT ID FROM {$wpdb->posts} WHERE guid='$attachment_url'";
	if($wpdb->get_var($query)){
		$attachment_id = $wpdb->get_var($query);
	}else{
		// If this is the URL of an auto-generated thumbnail, get the URL of the original image
		$attachment_url = preg_replace( '/-\d+x\d+(?=\.(jpg|jpeg|png|gif)$)/i', '', $attachment_url );
		//then run the query again
		$query = "SELECT ID FROM {$wpdb->posts} WHERE guid='$attachment_url'";		
		if($wpdb->get_var($query)){
			$attachment_id = $wpdb->get_var($query);
		}
	} 
	return $attachment_id;
}