Since you have defined the featured image for each post with using
<?php the_post_thumbnail( 'custombig' ); // or another custom size name ?>
should be enough, for info you can read:
http://markjaquith.wordpress.com/2009/12/23/new-in-wordpress-2-9-post-thumbnail-images/
Or here there is good info too:
How wordpress handle upload images and how to use them in the code