<div class="author-image">
<?php
$get_author_id = get_the_author_meta('ID');
$get_author_gravatar = get_avatar_url($get_author_id, array('size' => 300));
if(has_post_thumbnail()){
the_post_thumbnail();
} else {
echo '<img src="'.$get_author_gravatar.'" alt="'.get_the_title().'" />';
}
?></div>
