add-new-custom-avatar.php

<?php
/**
 * This function will add new custom avatar to WordPress.
 *
 * @param array $avatars List of avatars in WordPress.
 *
 * @author Mehul Gohil
 * @link https://www.mehulgohil.in/set-default-avatar/
 *
 * @return array
 *
 */
function mg_add_new_custom_avatar( $avatars ) {

	$new_avatar             = get_template_directory_uri() . '/assets/images/custom-gravatar.svg';
	$avatars[ $new_avatar ] = __( 'My Custom Gravatar', 'my-text-domain' );

	return $avatars;
}

add_filter( 'avatar_defaults', 'mg_add_new_custom_avatar', 10, 1 );