Thumbnail to external link AND Thumbnail to post - php

Question

Here is what I'd like to accomplish
Goal A: I want to hyperlink each thumbnail in index.php to their post.
Goal B: I want to define a hyperlink for each thumbnail in single.php to an external website.
You may ask why am I using thumbnails for single.php? The reason is because I want this layout:
And so far I understand that there are 3 methods to display images:
Insert image into the editor area along with the text, but the problem is I cannot float the image and text differently because all items within a post are assigned a p tag - am I wrong?
Custom fields should get the job done but it doesn't seem the most efficient way - or am I wrong?
Post Thumbnails should be the easiest way but see my problem below
I have the code to accomplish Goal A and B but they only work separately.
In other words, "Code 1" does not work if "Code 2" is present.
How can I resolve this issue? Or is there a better method accomplish my goal?
Code 1: Link thumbnails to external websites using custom field (single.php)
<?php $name = get_post_meta($post->ID, 'externalurl', true);
if( $name ) { ?>
<?php the_post_thumbnail(); ?>
<?php } else {
the_post_thumbnail();
} ?>
Code 2: Link thumbnails to the post (functions.php)
add_filter( 'post_thumbnail_html', 'my_post_image_html', 10, 3 );
function my_post_image_html( $html, $post_id, $post_image_id ) {
$html = '' . $html . '';
return $html;
}

Answer 1

is_single() function will help you achieve what you need. Try below code in functions.php and remove the additional code from single.php
function my_post_image_html( $html, $post_id, $post_image_id ) {
if ( is_single()) {
$name = get_post_meta($post_id, 'externalurl', true);
if( $name ) {
$html = '' . $html . '';
}
return $html;
}
else
{
$html = '' . $html . '';
return $html;
}
}
add_filter( 'post_thumbnail_html', 'my_post_image_html', 10, 3 );