If you followed guides for the new post thumbnail feature before WordPress 2.9 is out, they might tell you to paste this codes (or similar) into The Loop.

<?php if ( has_post_image() ) { ?>
	<a href="<?php the_permalink(); ?>"><?php the_post_image( array( 125, 125 ), array( 'class' => 'alignleft' ) ); ?></a>
<?php } ?>

However, the function names above were based on wordpress 2.9 RC1. The functions names actually changed when WordPress 2.9 is out.

• has_post_image() -> has_post_thumbnail()
• the_post_image() -> the_post_thumbnail()

So the codes you need to use is:

<?php if ( has_post_thumbnail() ) { ?>
	<a href="<?php the_permalink(); ?>"><?php the_post_thumbnail( array( 125, 125 ), array( 'class' => 'alignleft' ) ); ?></a>
<?php } ?>

That’s it.