This seems ridiculous, but this is what I’ve used:
<?php
if ( has_post_thumbnail()) {
$full_image_url = wp_get_attachment_image_src( get_post_thumbnail_id(), 'full');
echo '<img src="' . $full_image_url[0] . '" title="' . the_title_attribute('echo=0') . '" />';
}
?>
I’m not sure if it will work the same for get_post_thumbnail.