Typically, you would use the get_avatar filter.

Here’s a code example:

function rdp_wps_get_avatar_filter($avatar, $id_or_email, $size, $default, $alt){
    $user = null;
    $id = null;
    
    if ( is_numeric( $id_or_email ) ) {

        $id = (int) $id_or_email;
        $user = get_user_by( 'id' , $id );

    } elseif ( is_object( $id_or_email ) ) {

        if ( ! empty( $id_or_email->user_id ) ) {
            $id = (int) $id_or_email->user_id;
            $user = get_user_by( 'id' , $id );
        }

    } else {
        $user = get_user_by( 'email', $id_or_email );
    }    
    

    if($user && is_object( $user )){
        $profilePicURL = get_user_meta($user->ID, 'rdp_ll_picture_url',true);
        if(empty($profilePicURL))return $avatar;
        $avatar = sprintf('<img src="%s" class="avatar user-%2$d-avatar avatar-%4$d photo" width="%4$d" height="%4$d" alt="Profile photo of %3$s" />', esc_attr($profilePicURL),esc_attr($user->ID), esc_attr($user->display_name),esc_attr($size));       
    }
    return $avatar;
}
add_filter('get_avatar', 'rdp_wps_get_avatar_filter', 10, 5);