Adding a hook to a function

OK, I need to edit the code that is inserted by the get_image_tag function in the wp-includes/media.php file. I am adding this to the functions.php file in my theme but it’s not working. I could use a little direction here.

function myget_image_tag($id, $alt, $title, $align, $size='medium') {

	list( $img_src, $width, $height ) = image_downsize($id, $size);
	$hwstring = image_hwstring($width, $height);

	$class = 'align' . esc_attr($align) .' size-' . esc_attr($size) . ' wp-image-' . $id;
	$class = apply_filters('get_image_tag_class', $class, $id, $align, $size);

	$html = 'testkim<img src="' . esc_attr($img_src) . '" alt="' . esc_attr($alt) . '" title="' . esc_attr($title).'" '.$hwstring.'class="'.$class.'" rel="lightbox"/>';

	$html = apply_filters( 'get_image_tag', $html, $id, $alt, $title, $align, $size );

	return $html;
}

add_action ( 'get_image_tag', 'myget_image_tag($id, $alt, $title, $align, $size="medium")' );