the_post_thumbnail issues

I am using this code in my child theme to render thumbnail images for an image gallery:

<!-- Content of the slider Start  -->

		<?php rewind_posts(); $i = 1; while (have_posts()) : the_post(); ?>

			<div id="fragment-<?php echo $i; ?>" class="ui-tabs-panel" style="">

			<?php if((function_exists('has_post_thumbnail')) && (has_post_thumbnail())) : ?>

			<a href="<?php the_permalink(); ?>" rel="bookmark" title="<?php the_title(); ?>"><?php the_post_thumbnail( array(600,450));  ?></a>

			<?php endif; // endif post thumbnail ?>

				<div class="info" >

					<h2><a href="#" ><?php the_title(); ?></a></h2>
					<div id="slide-excerpt"><?php the_excerpt(); ?></div>
					<div id="slide-readmore"><a href="<?php the_permalink(); ?>" ><img src="<?php echo CHILDTEMPLATEIMAGES.'/main-slide-readmore.png'; ?>" alt="readmore" >  </img></a></div>

				</div> 

			</div> 

		 <?php $i++; endwhile; ?> 

		  <!-- Content of the slider end  -->

Somehow the images are rendered as 500 by 337 pixels and because of that I have an ugly empty space of 100 pixels. What am I missing here? Why are they not resized properly? The original images are bout 700 pixels by 450 pixels so they should do..