You are showing a small image stretched out.

the image is 840×557 and is being displayed at 1325×2000

so yes it will be blurry or pixelated.

Thanks for your response!

Yes I created a child theme. How can I rectify this problem? As far as I can see I have done everything to change the settings to allow images to be displayed as per their actual dimensions, what have I neglected to do?

Thanks

Nick

Thanks again for replying but as far as I’m aware I have already set the post thumbnail size correctly already, my functions.php file looks like this:

<?php
function my_theme_enqueue_styles() {

    $parent_style = 'parent-style'; // This is 'twentyfifteen-style' for the Twenty Fifteen theme.

    wp_enqueue_style( $parent_style, get_template_directory_uri() . '/style.css' );
    wp_enqueue_style( 'child-style',
        get_stylesheet_directory_uri() . '/style.css',
        array( $parent_style ),
        wp_get_theme()->get('Version')
    );
}
add_action( 'wp_enqueue_scripts', 'my_theme_enqueue_styles' );

set_post_thumbnail_size( 4000, 9999 );

?>

Do you think the fact that I added 9999 for unlimited height could be creating the problem? It seems to be the correct convention as per the instructions slightly further down the page you linked to.

Thanks again for your help!

Nick