If your using WP v3.5.1, you should use the 2012 theme, not 2010.

If you stick with the 2010 theme, you have to do more then just add code to the loop.php because thumbnail functionality is already built into the 2010 theme.
It grabs the thumb from the header.

To change that behavior, you might want to read this post and view this page on the WordPress codex for official reference.

i really advice to Upgrade your things and Follow the 2013 Features... But what David Advised is the best things to do if you would like to Stick with old things.