[HELP] WordPress Thumbnail add action if no thumbnail?

I was learning how to make my own WordPress plugin, currently I’m confusion how if there is no thumbnail in the post then I use the default image as thumbnail. code that I use:

add_action( 'the_post', 'mythumb' );
function mythum(){
   if (!has_post_thumbnail()) {
    $defaultthum = "http://example.com/default.jpg"
    echo ('<div class="featured-thumbnail"><img width="150" height="150" src="'.$defaultthum.'" class="attachment-featured wp-post-image" alt="7" title="" /></div>')
   }
}

my problem: the default image thumbnail is not in the right position. see this picture: http://s22.postimg.org/ezt05f59d/Lm_XUp.png any idea? or the hooks is not the_post?
Thanks