This snippet allows users to upload a custom avatar image.

custom-gravatar.php

<?php
/**
 * Custom Gravatar
 *
 * This snippet adds a custom gravatar field to the user profile page.
 * It also adds a filter to replace the default gravatar with the custom one.
 * To make it work, you need to have Advanced Custom Fields plugin installed and activated.
 *
 * Usage:
 * 1. Copy this snippet to your theme's directory (e.g. /wp-content/themes/your-theme/inc/custom-gravatar.php)
 * 2. Include this snippet in your functions.php file with the following code:
 *    require_once get_template_directory() . '/inc/custom-gravatar.php';
 * 3. Add the following code to your theme's template file where you want to display the gravatar:
 *    echo get_avatar(get_the_author_meta('ID'), 96);
 * 4. Go to the user profile page in the admin panel and upload a custom gravatar image.
 * 5. Refresh the page where you added the code from step 3.
 * 6. The default gravatar should be replaced with the custom one.
 *
 * @author Alex Zappa <alex@zappa.dev>
 * @copyright 2024 Alex Zappa
 */

if (!defined('ABSPATH'))
    exit;

if (class_exists('ACF')) {

    add_action('acf/include_fields', function () {
        
        if (!function_exists('acf_add_local_field_group')) {
            return;
        }

        acf_add_local_field_group(array(
            'key' => 'user_extra_options',
            'title' => 'User Extra Options',
            'fields' => array(
                array(
                    'key' => 'user_extra_options__custom_gravatar',
                    'label' => 'Custom Gravatar',
                    'name' => 'custom_gravatar',
                    'aria-label' => '',
                    'type' => 'image',
                    'instructions' => '',
                    'required' => 0,
                    'conditional_logic' => 0,
                    'wrapper' => array(
                        'width' => '',
                        'class' => '',
                        'id' => '',
                    ),
                    'return_format' => 'id',
                    'library' => 'all',
                    'min_width' => '',
                    'min_height' => '',
                    'min_size' => '',
                    'max_width' => '',
                    'max_height' => '',
                    'max_size' => '',
                    'mime_types' => '',
                    'preview_size' => 'thumbnail',
                ),
            ),
            'location' => array(
                array(
                    array(
                        'param' => 'user_form',
                        'operator' => '==',
                        'value' => 'all',
                    ),
                ),
            ),
            'menu_order' => 0,
            'position' => 'normal',
            'style' => 'default',
            'label_placement' => 'top',
            'instruction_placement' => 'label',
            'hide_on_screen' => '',
            'active' => true,
            'description' => '',
            'show_in_rest' => 0,
        ));
    });

    function sunnypixels_custom_avatar($avatar, $id_or_email, $size, $default, $alt, $args)
    {
        $id = sunnypixels_get_user_id($id_or_email);

        if (!$id)
            return $avatar;

        $custom_avatar = get_user_meta($id, 'custom_gravatar', true);
        if ($custom_avatar) {
            $custom_avatar = wp_get_attachment_image_src($custom_avatar)[0];
            $avatar = "<img alt='{$alt}' src='{$custom_avatar}' class='avatar avatar-{$size} photo' height='{$size}' width='{$size}' {$args['extra_attr']} />";
            //php_pretty_print($avatar);
        }
        return $avatar;
    }

    function sunnypixels_custom_avatar_url($url, $id_or_email, $args)
    {
        $id = sunnypixels_get_user_id($id_or_email);

        if (!$id)
            return $url;

        $custom_avatar = get_user_meta($id, 'custom_gravatar', true);
        if ($custom_avatar) {
            $custom_avatar = wp_get_attachment_image_src($custom_avatar)[0];
            $url = $custom_avatar;
        }
        return $url;
    }

    function sunnypixels_get_user_id($id_or_email)
    {
        if (is_numeric($id_or_email))
            return $id_or_email;

        if (is_object($id_or_email))
            return $id_or_email->user_id;

        if (is_string($id_or_email))
            return get_user_by('email', $id_or_email) ? get_user_by('email', $id_or_email)->ID : 0;

        return 0;
    }

    add_filter('get_avatar', 'sunnypixels_custom_avatar', 100500, 6);
    add_filter('get_avatar_url', 'sunnypixels_custom_avatar_url', 100500, 3);
    
} else {
    
    add_action('admin_notices', function () {
        echo '<div class="notice notice-warning is-dismissible"><p>Advanced Custom Fields plugin is required for Custom Gravatar functionality.</p></div>';
    });

}

Check out the new variation of the plugin too!