wp-post-thumbnail classes

Is there a simple explanation to styling featured image thumbnails other than having two classes get parsed in the browser? Seems really inefficient. I’d place a simple styling in the CSS, but it then becomes site wide and I have a few scenarios of featured image styles.

My code looks like this:

<div>
			<a href="<?php the_permalink(); ?>" title="<?php printf( __('Permalink to %s', 'doghouse'), the_title_attribute('echo=0') ); ?>" rel="bookmark"><?php the_post_thumbnail(); ?></a><?php the_excerpt();?>
		</div>

The CSS looks like this now:

.attachment-post-thumbnail{
	display: inline;
	float:left;
	margin-right: 1em;
	border: 1px solid #000000;

}

Thanks.