if you add this before the li tag:
<?php
echo '<pre>';
echo htmlspecialchars(print_r($image,true));
echo '</pre>';
?>
you will see the sizes that are defined. i assume full is not one of them. 😉
you can add additional sizes with plugin or inside function.php
add_image_size( 'myfullsize', '4000', '3000', false );
but i assume you just need to change a line of your code.
change it to this:
<img src="<?php echo $image['url']; ?>" alt="<?php echo $image['alt']; ?>" />
