Just delete your timthumb.php file.
If your site thumbnails use timthumb then the code might look like this
<img src="https://wordpress.stackexchange.com/questions/80287/<?php bloginfo("template_directory'); ?>/xxxxx/timthumb.php?src=<?php echo "original image url" ?>&h=150&w=150&zc=1" />
Just replace those instances with code like this
the_post_thumbnail('thumbnail', array('class' => 'thubnail'));