Use Gravatar images in WordPress but don't reveal the Gravatar img src URL. Display the Gravatar inline instead.

not-gravatar.php

<?php
/*
Plugin Name: Tin Foil Hat Gravatars
Description: Use Gravatars but don't use Gravatars. What? Shut up.
Author: Jan Dembowski
Author URI: https://blog.dembowski.net/
Version: 0.7
License: GPL2
*/

add_filter( 'get_avatar', 'tinfoilhat_get_avatar', 10, 6 );

function tinfoilhat_get_avatar( $avatar, $id_or_email, $size, $default, $alt, $args ) {

	// Are we in the WordPress dashboard? If so stop, don't
	// bother with any Gravatar substitutions.

	if ( is_admin() ) return $avatar;

	// Save the orginal hash of the unmodified $avatar
	$mh_avatar_hash = md5( $avatar );

	// Is the result already in a transient? If so get and return that.
	$mh_transient_avatar = get_transient( 'mh_notgravatar_' . $mh_avatar_hash );

	if( $mh_transient_avatar ) {
		$avatar = '<!-- Transient Gravatar -->' . $mh_transient_avatar;
		return $avatar;
	}

	// If there's no Gravatar to hide then just return $avatar after
	// the email hash is munged.

	if( !(mh_validate_gravatar( $id_or_email ))) {

		// Get md5 for example+number@example.com
		$mh_email_hash = md5( strtolower( trim( 'example+' . rand(1,2000) . '@example.com' )));

		// Replace the current md5 with the new one
		$avatar = preg_replace( '/gravatar.com\/avatar\/.{32}/', 'gravatar.com/avatar/' . $mh_email_hash, $avatar );

		// Save the new $avatar in a transient for the next time
		set_transient( 'mh_notgravatar_' . $mh_avatar_hash, $avatar, 30 * MINUTE_IN_SECONDS );

		return $avatar;
	}

	// If there is a valid Gravatar then remove the URL and inline the image.

	// Regex and preg_match_all to put all urls from $avatar into an array.
	$mh_src_regex = "/'(http|https)\:(.*?)[' ]/";
	preg_match_all( $mh_src_regex , $avatar, $mh_urls );

	// Go through that array and replace src='url' with src='data:...'
	foreach( $mh_urls[0] as $match ) {

		// Remove the first and last characters in the string. The regex 
		// is close enough for government work but has 1 extra character 
		// at the start and end.
		$mh_old_url = substr( $match , 1, -1 );

		// Download the image into an array.
		$mh_image = wp_remote_get( $mh_old_url );

		// base64 encode the image from the body part of that download.
		$mh_inline_gravatar = base64_encode( $mh_image['body'] );

		// I need the content type from the headers for JPEG/PNG or
		// whatever was retrieved.
		$mh_content_type = $mh_image['headers']['content-type'];

		// Put it together and make the inline substitution for the avatar
		$mh_inline_src = 'data:' . $mh_content_type . ';base64,' . $mh_inline_gravatar;
		$avatar = str_replace( $mh_old_url, $mh_inline_src, $avatar );
	}

	// Save the new $avatar in a transient for the next time
	set_transient( 'mh_notgravatar_' . $mh_avatar_hash, $avatar, 30 * MINUTE_IN_SECONDS );

	return $avatar;
}

// I copied this whole from https://gist.github.com/justinph/5197810
// Justin Heideman (https://github.com/justinph) wrote validate_gravatar 
// and it's very complete. The wp_cache part is neat.
//
// I prefixed the function name in case a validate_gravatar() exists
// in future WordPress versions.

function mh_validate_gravatar($id_or_email) {
  //id or email code borrowed from wp-includes/pluggable.php
	$email = '';
	if ( is_numeric($id_or_email) ) {
		$id = (int) $id_or_email;
		$user = get_userdata($id);
		if ( $user )
			$email = $user->user_email;
	} elseif ( is_object($id_or_email) ) {
		// No avatar for pingbacks or trackbacks
		$allowed_comment_types = apply_filters( 'get_avatar_comment_types', array( 'comment' ) );
		if ( ! empty( $id_or_email->comment_type ) && ! in_array( $id_or_email->comment_type, (array) $allowed_comment_types ) )
			return false;

		if ( !empty($id_or_email->user_id) ) {
			$id = (int) $id_or_email->user_id;
			$user = get_userdata($id);
			if ( $user)
				$email = $user->user_email;
		} elseif ( !empty($id_or_email->comment_author_email) ) {
			$email = $id_or_email->comment_author_email;
		}
	} else {
		$email = $id_or_email;
	}

	$hashkey = md5(strtolower(trim($email)));
	$uri = 'http://www.gravatar.com/avatar/' . $hashkey . '?d=404';

	$data = wp_cache_get($hashkey);
	if (false === $data) {
		$response = wp_remote_head($uri);
		if( is_wp_error($response) ) {
			$data = 'not200';
		} else {
			$data = $response['response']['code'];
		}
	    wp_cache_set($hashkey, $data, $group = '', $expire = 60*5);

	}		
	if ($data == '200'){
		return true;
	} else {
		return false;
	}
}