How to resize image, not crop on post thumb

I use this script on funtion.php to retrieve first image of post on home page to show as thumbnail, and the script will work same as i want, but the post image will crop not resize. can anyone tell me what modification i do in this code, so the image will resize.

// Retrieve the src of first image in a post
function bm_extract_string($start, $end, $original) {
$original = stristr($original, $start);
$trimmed = stristr($original, $end);
return substr($original, strlen($start), -strlen($trimmed));
}
function getFirstImage() {
$content = get_the_content();
$pic_string = bm_extract_string('src="','" ',$content);
$imagesize = getimagesize($pic_string);
list($width, $height, $type, $attr) = getimagesize($pic_string);
$link = get_permalink();
$title = get_the_title($post->post_title);
echo '<a href="'.$link.'" style="background:url('.$pic_string.'); display:block; width:'.$width.'px; height:'.$height.'px;" title="'.$title.'"></a>';
}

than i add this code in index.php

<?php getFirstImage(); ?>

this is my site : https://www.movieflake.com/ (check home page, all images are crop, not resize)