too long

The thumbnails of all my Wordpress pages are exactly the same--an off-centered logo--when I share the links on Kakaotalk (Popular SNS in South Korea). I suspect this image comes from when I changed the wordpress login logo to a custom logo by messing with the functions.php

//Custom login logo

function my_login_logo() { ?>
    <style type="text/css">
        #login h1 a, .login h1 a {
            background-image: url(<?php echo get_stylesheet_directory_uri(); ?>/images/site-login-logo.png);
        }
    </style>
<?php }
add_action( 'login_enqueue_scripts', 'my_login_logo' );

I would like to change all the thumbnails to a better looking image, but all the tutorials I've found are for changing thumbnails of wordpress posts, not pages. Is there a php function that can change the thumbnails of all wordpress pages?

The thumbnail was coming from a png logo in my homepage. It wasn't off-centered, the text next to the black logo is white so it only looks off-centered. Thanks to ProEvilz for the tip of Facebook Debugger. I just changed the png logo to a jpg with blue background, so the thumbnail has some blue color now so I can see the white text next to the black logo