CroiSciento on "Post thumbnail in $_POST array?"

Hello,

I'm making a basic php basket and i'm trying to retrieve different informations. So far so good I got the title, the content, the permalink everything is working at intended. But i'd like to save the_post_thumbnail in an input hidden like this :

<input type='hidden' name='thumbnail_image' value="<?php the_post_thumbnail(); ?>" />

The problem is that it returns me a number. But I've basically never done this in the past even with a basic image. So how should I proceed save this picture in my array?

Thanks for your help.