Get most recent media upload

I don’t think that you can use get_the_post_thumbnail function to get last uploaded media, but you can use get_post to get latest attachment and then wp_get_attachment_image to display images.

$attachments = get_posts( array(
    'post_type' => 'attachment',
    'posts_per_page' => 1,
    'post_status' => null,
    'post_mime_type' => 'image'
) );

foreach ( $attachments as $attachment ) {
    echo wp_get_attachment_image( $attachment->ID, 'thumbnail' );
}