Add alt tag to WordPress Gravatar images

functions.php

<?php

// Add alt tag to WordPress Gravatar images
// Full how-to: https://bloggerpilot.com/gravatar-alt-tag/
function bloggerpilot_gravatar_alt($bloggerpilotGravatar) {
	if (have_comments()) {
		$alt = get_comment_author();
	}
	else {
		$alt = get_the_author_meta('display_name');
	}
	$bloggerpilotGravatar = str_replace('alt=\'\'', 'alt=\'Avatar f&uuml;r ' . $alt . '\'', $bloggerpilotGravatar);
	return $bloggerpilotGravatar;
}
add_filter('get_avatar', 'bloggerpilot_gravatar_alt');

You need to add it to your functions.php files in your child theme, or to a snippet plugin.