the codes listed here is part of the following article : https://www.mwordpress.net/add-thumbnail-to-wordpress-without-custom-field/

auto_set_featured.php

/*
 * Get attachment id for default image url
 * $default url
*/
function get_attachment_id_from_src($image_src) {
	global $wpdb;
	$query = "SELECT ID FROM {$wpdb->posts} WHERE guid='$image_src'";
	$id = $wpdb->get_var($query);
	return $id;
}
/*
 * Auto set featured image
 * Note : image must be in post attachemnts
*/
function autoset_featured() {
	global $post;
	$postsetid = get_the_ID();
	$already_has_thumb = has_post_thumbnail($post->ID);
	$default = 'http://link-to-image’;

	if (!$already_has_thumb)  {
		$attached_image = get_children('post_type=attachment&post_mime_type=image&post_parent='.$postsetid );
		if ($attached_image) {
			foreach ($attached_image as $attachment_id => $attachment) {
				set_post_thumbnail($post->ID, $attachment_id);
			}
		} else {
			set_post_thumbnail($post->ID, get_attachment_id_from_src(get_attachment_id_from_src($default)));
		}
	}
}
add_action('the_post', 'autoset_featured');

getImage.php

/*
 * Get First Image in post content
 * Note : the post content must contain one image at least
*/
function getImage($num) {
	global $more;
	$more 		= 1;
	$content 	= get_the_content();
	$count 		= substr_count($content, '<img');
	$start 		= 0;
	for($i=1;$i<=$count;$i++) {
		$imgBeg = strpos($content, '<img', $start);
		$post = substr($content, $imgBeg);
		$imgEnd = strpos($post, '>');
		$postOutput = substr($post, 0, $imgEnd+1);
		$image[$i] = $postOutput;
		$start=$imgEnd+1;

		$cleanF = strpos($image[$num],'src="')+5;
		$cleanB = strpos($image[$num],'"',$cleanF)-$cleanF;
		$imgThumb = substr($image[$num],$cleanF,$cleanB);
	}
	if(stristr($image[$num],'<img')) {
		echo $imgThumb;
	}
	$more = 0;
}

image.html

<div class="thumbnail">
	<a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>">
		<img src="<?php bloginfo('template_directory'); ?>/timthumb.php?src=<?php getImage('1'); ?>&w=150&h=150&zc=1">
	</a>
</div>

style.css

.thumbnail a:link,
.thumbnail a:visited {
	display:block;
	float:left;
	padding:5px;
	background:#e2e2e2;
	width:150px;
	height:150px;
	margin:5px 5px 0 0;
}
.thumbnail a:hover,
.thumbnail a:active {
	background:#C4C4C4;
}