How to add thumbnail images to posts and pages in an elegant way?

Hello! please i am wondering if there was a plugin or hack that enables me to append a nice looking thumbnail for each post or page i publish on my blog.

Currently, i am able to add the featured images but appending those images to my posts using the below code is not getting me any tidy results.

<?php
if ( has_post_thumbnail() ) {
the_post_thumbnail();
}  ?>

I am mainly looking for something simple that would fix those thumbnails automatically and uniformly in my posts or pages without adding them manually and adjusting their sizes and positions within the text one by one. As if i have 500+ posts, i have to repeat the process 500 times spending long hours adjusting each single image in my posts which is literally very time consuming. At the moment i have them set already as featured images but not displayed inside the posts or pages.

Please check the below link for an example of a tidy thumbnail:
http://www4.picturepush.com/photo/a/12511612/img/12511612.jpg

Thanks in advance for your help! any suggestions are greatly appreciate it.