Hello,

I know this is an old thread but it showed up on Google when I searched for the same possibility.
I figured to make it work : display a 2:3 ratio image to Pinterest share pins.

First, you need to create a custom size with 2:3 ratio, here I took 600×900 :

add_image_size( 'pinterest-size', 600, 900, true );
add_filter( 'image_size_names_choose', 'custom_pinterest_size' );
function custom_pinterest_size( $sizes ) {
    return array_merge( $sizes, array(
        'pinterest-size' => __( 'Ratio 2:3' ),
    ) );
}

Then, you can edit the pinterest URL pattern with avia_add_social_share_link_arguments :

add_filter('avia_social_share_link_arguments', 'avia_add_social_share_link_arguments', 10, 1);
function avia_add_social_share_link_arguments($args)
{
	if(is_single() && get_the_ID()){
		$args['pinterest'] = array("encode"=>true, "encode_urls"=>true, "pattern" => "https://pinterest.com/pin/create/button/?url=[permalink]&description=[title]&media=".get_the_post_thumbnail_url(get_the_ID(), 'pinterest-size'), 'label' => __("Share on Pinterest",'avia_framework'));
	}
	return $args;
}

And that’s it !
Hope it will help some other who get to this thread from Google with the same issue.