Need to make my home page featured images clickable to full post page

Do you have a homepage template file? Have you tried to
<?php echo 'Testing to see if this is the active template'; ?>
in various templates to see which one is being used? Once you find the tpl, then you can do something like this, in it:

<a href="<?php echo get_attachment_link(get_post_thumbnail_id($post->ID)) ?>">
<?php the_post_thumbnail('feat-img); ?></a>

Thanks! The content.php that I took the above code from is the active template for my homepage. I just couldn’t figure out where to wrap the code with . See here:

<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
<?php
if ( '' != get_the_post_thumbnail() ) :
$image = wp_get_attachment_image_src( get_post_thumbnail_id(), 'feat-img' );

// If the image is greater than or equal to 645, show it
if ( $image[1] >= 645 ) :
the_post_thumbnail( 'feat-img' );
endif;
endif;
?>

Replace:

<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
<?php
if ( '' != get_the_post_thumbnail() ) :
$image = wp_get_attachment_image_src( get_post_thumbnail_id(), 'feat-img' );

// If the image is greater than or equal to 645, show it
if ( $image[1] >= 645 ) :
the_post_thumbnail( 'feat-img' );
endif;
endif;
?>

With this:

<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
<?php
if ( '' != get_the_post_thumbnail() ) {
$image = wp_get_attachment_image_src( get_post_thumbnail_id(), 'feat-img' );

// If the image is greater than or equal to 645, show it
if ( $image[1] >= 645 ) { ?>

<a href="<?php echo get_attachment_link(get_post_thumbnail_id($post->ID)) ?>">
<?php the_post_thumbnail('feat-img); ?></a>

<?php
}
}
?>

Thnak you. When I do that, tho, I am getting a syntax error. Is something wrong with this line: <?php the_post_thumbnail('feat-img); ?></a> ?

That’s where it seems to start.
Here’s the whole content.php

<?php
/**
 * @package Superhero
 * @since Superhero 1.0
 */
?>

<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
<?php
if ( '' != get_the_post_thumbnail() ) {
$image = wp_get_attachment_image_src( get_post_thumbnail_id(), 'feat-img' );

// If the image is greater than or equal to 645, show it
if ( $image[1] >= 645 ) { ?>

<a href="<?php echo get_attachment_link(get_post_thumbnail_id($post->ID)) ?>">
<?php the_post_thumbnail('feat-img); ?></a>

<?php
}
}
?>

	<header class="entry-header">
		<h1 class="entry-title"><a href="<?php the_permalink(); ?>" rel="bookmark"><?php the_title(); ?></a></h1>

		<?php if ( 'post' == get_post_type() ) : ?>
		<div class="entry-meta">
			<?php superhero_posted_on(); ?>
		</div><!-- .entry-meta -->
		<?php endif; ?>
	</header><!-- .entry-header -->

	<?php if ( is_search() ) : // Only display Excerpts for Search ?>
	<div class="entry-summary">
		<?php the_excerpt(); ?>
	</div><!-- .entry-summary -->
	<?php else : ?>
	<div class="entry-content">
		<?php the_content( __( 'Continue reading <span class="meta-nav">&rarr;</span>', 'superhero' ) ); ?>
		<?php wp_link_pages( array( 'before' => '<div class="page-links">' . __( 'Pages:', 'superhero' ), 'after' => '</div>' ) ); ?>
	</div><!-- .entry-content -->
	<?php endif; ?>

	<footer class="entry-meta">
		<?php if ( 'post' == get_post_type() ) : // Hide category and tag text for pages on Search ?>
			<?php
				/* translators: used between list items, there is a space after the comma */
				$categories_list = get_the_category_list( __( ', ', 'superhero' ) );
				if ( $categories_list && superhero_categorized_blog() ) :
			?>
			<span class="cat-links">
				<?php printf( __( 'Posted in %1$s', 'superhero' ), $categories_list ); ?>
			</span>
			<?php endif; // End if categories ?>

			<?php
				/* translators: used between list items, there is a space after the comma */
				$tags_list = get_the_tag_list( '', __( ', ', 'superhero' ) );
				if ( $tags_list ) :
			?>
			<span class="sep"> | </span>
			<span class="tags-links">
				<?php printf( __( 'Tagged %1$s', 'superhero' ), $tags_list ); ?>
			</span>
			<?php endif; // End if $tags_list ?>
		<?php endif; // End if 'post' == get_post_type() ?>

		<?php if ( ! post_password_required() && ( comments_open() || '0' != get_comments_number() ) ) : ?>
		<span class="sep"> | </span>
		<span class="comments-link"><?php comments_popup_link( __( 'Leave a comment', 'superhero' ), __( '1 Comment', 'superhero' ), __( '% Comments', 'superhero' ) ); ?></span>
		<?php endif; ?>

		<?php edit_post_link( __( 'Edit', 'superhero' ), '<span class="sep"> | </span><span class="edit-link">', '</span>' ); ?>
	</footer><!-- .entry-meta -->
</article><!-- #post-## -->

Yeah, sorry – I missed the ending single quote.
This:
<?php the_post_thumbnail('feat-img); ?></a>
Should be this:
<?php the_post_thumbnail('feat-img'); ?></a>

Aha!! Good eye.
So this works, but it takes you to the attachment image, not the corresponding post.

Thanks for helping me. I appreciate this SO MUCH!

This should link the image to the post to which it is attached:

<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
<?php
if ( '' != get_the_post_thumbnail() ) {
$image = wp_get_attachment_image_src( get_post_thumbnail_id(), 'feat-img' );

// If the image is greater than or equal to 645, show it
if ( $image[1] >= 645 ) { ?>

<a href="<?php echo get_permalink($post->ID) ?>">
<?php the_post_thumbnail('feat-img'); ?></a>

<?php
}
}
?>

Perfect! Thank you so much!!!!!! Can I buy you a cup of coffee??

Thank you kindly, Sam – no need for the cup of coffee – glad to help.