I have this code in my child theme but it's pulling in the category featured image instead of the individual single posts image, when viewing a list of single post under category. The code is within a loop.
<?php if (has_post_thumbnail()) { ?>
<?php the_post_thumbnail( "full" ); ?>
<?php } ?>
If anybody gets caught up on retreiving an image outside of a single post this solution worked for me.
<img src="<?php $image_source = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'full' ); echo $image_source[0]; ?>">
The $post->ID may have to change depending on what you're working on.