Changing post thumbnail size

Hi,

Can anyone tell me please how can I change seize in post thumbnail please?

I have this in my loop.php
<?php the_post_thumbnail(array(500,100)); ?>

so what i am trying to achieve is to crop any image to size width 500 ps and height 100px but it is keep on resizing image to 100 x 100.

many thanks for your help in advance.

dompl