Aligning the thumbnail image

I want to control the alignment of the featured image(the thumbnail) with the options given in the ‘Set featured image’ pop box.

<?php if(has_post_thumbnail()) {
   echo '<div class="border">'; the_post_thumbnail(); echo '</div>';
} ?>

I want to add to the div with the class of border a class floatleft or floatright(or nothing) according for what the user chooses.

Thanks!