WordPress problem with force image resize using timthumb?

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Now i am using Timthum for resize the feature images, i want to resize with fixed width and height, bu开发者_如何学编程t here the image size was crop, here is my code.

<?php 
       $src = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), array( 300,300 ), false, '' ); 

     $thumbnailSrc = $src[0]; 

?>
<img src="<?php bloginfo('template_directory'); ?>/timthumb.php?src=<?php echo $thumbnailSrc; ?>&h=256&w=400" alt="" />