Allows for custom avatars to be pulled from usermeta in WordPress

custom_user_avatar_url.php

<?php
// Allows for custom avatars to be pulled from usermeta
add_filter('get_avatar_url', 'custom_user_avatar_url', 10, 3);
function custom_user_avatar_url($url, $id_or_email, $args) {
  $image_url = false;
  $user = false;

  if (is_object($id_or_email) && isset($id_or_email->comment_ID)) {
    $id_or_email = get_comment($id_or_email);
  }

  if (is_numeric($id_or_email)) {
    $user = get_user_by('id', absint($id_or_email));
  } elseif (is_string($id_or_email)) {
    if (strpos($id_or_email, '@md5.gravatar.com')) {
      return $url;
    } else {
      $user = get_user_by('email', $id_or_email);
    }
  } elseif ($id_or_email instanceof WP_User) {
    $user = $id_or_email;
  } elseif ($id_or_email instanceof WP_Post) {
    $user = get_user_by('id', (int) $id_or_email->post_author);
  } elseif ($id_or_email instanceof WP_Comment) {
    if (!empty( $id_or_email->user_id)) {
      $user = get_user_by('id', (int) $id_or_email->user_id);
    }

    if ((!$user || is_wp_error($user)) && !empty($id_or_email->comment_author_email)) {
      $user = get_user_by('email', $id_or_email->comment_author_email);
    }
  }

  if (!$user) return $url;

  // You can change this variable name to $image_url if you are storing image URLs instead of IDs
  // Also, be sure to change the usermeta key to match whichever key you are storing the data
  $image_id = get_user_meta($user->ID, 'profile_image_id', true);

  if (
    !$image_url && $image_id &&
    $image = wp_get_attachment_image_src($image_id, array($args['width'], $args['height']))
  ) $image_url = $image[0];

  if ($image_url) return $image_url;

  return $url;
}
?>