Defining post ID via post_ID code?

Hi! I’m modifying some code and am wondering if I’m doing something wrong. Instead of having it use the date (in Y-m-d format), I want it to use the post ID. The code’s purpose is basically defining a header image dynamically, based on the post ID. It worked with the date code, but doesn’t work with the post_ID code.

Here’s the original code:
<?php the_time('Y-m-d') ?>

And here’s my modification:
<?php the_ID(); ?>

So the final code for the entire section would be:

<div id="comic">
<?php
error_reporting(0);
$gposts = get_posts('numberposts=1');
foreach ($posts as $post) :
?>

<?php echo '<img src="http://collegeandbeyond.org/headers/' ?>
<?php the_ID(); ?><?php echo '.jpg" alt="Comic" />' ?>

<?php
endforeach;
?>
</div>

Any ideas?

Thanks for the help!