I got it now using the following code:
//featured image from postmeta
function postmeta_img_update() {
global $post;
// If Custom Post Meta Field for a Image/Thumnail Exists
if( get_post_meta($post->ID, "_image", true):
// Get Image Location and File Extention
$img = get_post_meta($post->ID, "_image", true);
$ext = substr(strrchr($img,'.'),1);
// WordPress Upload Directory to Copy to (must be CHMOD to 777)
$uploads = wp_upload_dir();
$copydir = $uploads['path']."https://wordpress.stackexchange.com/";
// Code to Copy Image to WordPress Upload Directory (Server Must Support file_get_content/fopen/fputs)
$data = file_get_contents($img);
$filetitle = strtolower(str_replace(array(' ', '-', '.', '(', ')', '!', '@', '#', '$', '%', '^', '&', '*', '_', '=', '+', "https://wordpress.stackexchange.com/", '"'), "-", get_the_title()));
$file = fopen($copydir . "$filetitle-$post->ID.$ext", "w+");
fputs($file, $data);
fclose($file);
// Insert Image to WordPress Media Libarby
$filepath = $uploads['path']."/$filetitle-$post->ID.$ext";
$wp_filetype = wp_check_filetype(basename($filepath), null );
$attachment = array(
'post_mime_type' => $wp_filetype['type'],
'post_title' => get_the_title(),
'post_content' => 'Image for '.get_the_title(),
'post_status' => 'inherit'
);
wp_insert_attachment( $attachment, $filepath, $post->ID);
// Get Attachment ID for Post
global $wpdb; $attachment_id = $wpdb->get_var("SELECT ID FROM $wpdb->posts WHERE post_parent="$post->ID" AND post_status="inherit" AND post_type="attachment" ORDER BY post_date DESC LIMIT 1");
// Attached Image as Featured Thumbnail in Post
update_post_meta($post->ID, "_thumbnail_id", $attachment_id);
// Removes Custom Meta Field Image URL. This stop this function running again for the updated post.
delete_post_meta($post->ID, "_image");
endif;
}
add_action('the_post','postmeta_img_update');