This what <cite class="fn" title="http://justintadlock.com"></cite><a href="http://justintadlock.com/" title="Justin Tadlock" class="url" rel="external nofollow">Justin Tadlock</a> said in his blog post given as external resource on this page<br>
<a href="http://codex.wordpress.org/Function_Reference/the_post_thumbnail">http://codex.wordpress.org/Function_Reference/the_post_thumbnail</a><br><br><blockquote>Yep, <b>the resizing and cropping is definitely best done by TimThumb </b>at
 the moment.  The only alternate solution to that would be for a theme 
to set its own intermediate image sizes and have a button for users to 
click to generate those from old attachments.  I’m not sure how feasible
 it is to do this, but it’s something I plan on looking into.</blockquote><blockquote><p>Personally, I’ll probably keep using custom fields and the Get the 
Image script.</p></blockquote>
<br><a href="http://justintadlock.com/archives/2009/11/16/everything-you-need-to-know-about-wordpress-2-9s-post-image-feature#comment-162754">http://justintadlock.com/archives/2009/11/16/everything-you-need-to-know-about-wordpress-2-9s-post-image-feature#comment-162754</a>