How to reset post data properly

When using get_posts() you don’t actually setup post data unless you do so explicitly, via setup_postdata() inside your (usually a foreach) loop.

So, since you’re not setting up post data, you shouldn’t need to reset post data.

but still the output fails for all but the first post

I think that’s because you’re calling the global $post inside your function, but aren’t globalizing it first:

$image_id = get_post_thumbnail_id($post->ID);
$args = array(
    'post_type' => 'attachment',
    'post_status' => null,
    'post_parent' => $post->ID,
    'include'  => $image_id
    ); 
$image_data = get_posts($args);

Try adding global $post; before using $post->ID:

global $post;
$image_id = get_post_thumbnail_id($post->ID);
$args = array(
    'post_type' => 'attachment',
    'post_status' => null,
    'post_parent' => $post->ID,
    'include'  => $image_id
    ); 
$image_data = get_posts($args);

If that’s not the problem, can you clarify:

1. What data you get from $iamge_data
2. The context in which you are calling the function