WordPress Theme : Check for featured image

wp-featured-img-check.php

/*
Add the following to your theme's function.php file
Useful when you have multiple excerpt and/or content files
*/

// Check to see if the post has a featured image then output HTML
// Then add a class to via post_class if it does
function my_post_thumbnail($thumb_size = 'thumbnail') {
	if ( has_post_thumbnail() ) {
		ob_start();
	?>
		<a href="<?php the_permalink() ?>"><?php the_post_thumbnail($thumb_size); ?></a>
	<?php
		$thumb = ob_get_clean();
	}
	else {
		$thumb = '';
	}
	return $thumb;
}
function my_post_thumbnail_class($class = '') {
	if ( has_post_thumbnail() ) {
		$class[] = 'has-thumbnail';
	}
	return $class;
}
add_filter('post_class', 'my_post_thumbnail_class');

/*
Then add this to your template files
*/
echo my_post_thumbnail('enter-size-here');