Dave McCourt thinks some thoughts...

/*

    Member image uploads on front-end
    Source http://goldenapplesdesign.com/2010/07/03/front-end-file-uploads-in-wordpress/

*/

function banana__insert_attachment($file_handler, $post_id, $field_key = 'false') {
 
  // check to make sure it is a successful upload
  if ($_FILES[$file_handler]['error'] !== UPLOAD_ERR_OK) __return_false();
 
  require_once(ABSPATH . "wp-admin" . '/includes/image.php');
  require_once(ABSPATH . "wp-admin" . '/includes/file.php');
  require_once(ABSPATH . "wp-admin" . '/includes/media.php');
 
  $attach_id = media_handle_upload($file_handler, $post_id);

  if (isset($attach_id) && !is_wp_error($attach_id)) {
	  if ($field_key) update_field($field_key, $attach_id, $post_id);
	  return $attach_id;
  }
    
}

Set a featured image

$newupload = banana__insert_attachment($file, 0);
if ($newupload) {
   set_post_thumbnail($post_id, $newupload);
}

Attach an upload to a custom field

$newupload = banana__insert_attachment($file, 0);
if ($newupload) {
   update_field($field_key, $newupload, $post_id);
}