This plugin will scan your thumbnail-less posts and look for a suitable image to use as a thumbnail. 
It will look for images linked in the post content and will download and add them to the local library. 
Post scanning is performed only when <code>the_post_thumbnail</code> or <code>get_the_post_thumbnail</code> are called for 
that particular post.
So you designed your new thumbnail-based template, but you have tons of thumbnail-less posts and 
don’t want to spend too much time on scraping your old posts for a thumbnail picture. Maybe you 
think you will probably forget to set a thumbnail for your new posts. Maybe both, as was my case :).
When a thumbnail for a post is queried and nothing is found this plugin will scrape the post for 
an <code><img></code> tag and its <code>href</code> property. Once a suitabile image is found (based on the configured 
settings), it will be downloaded ad set as the post thumbnail. If no suitable image is found, 
a default one will be used.
This plugin is still in early development state. It just covers my needs (see the “To do” section 
for more details). It has been tested only with wordpress 3.0.1, but should work also with 2.9, 
since I used similar code with 2.9 (even if it was included in my template’s <code>functions.php</code>).
The plugin is not supported. This doesn’t mean that I will not look into submitted bugs. Anyway, 
don’t expect prompt answers, since this is done in my spare time.
<h3>To do</h3>
Many things should be done in future releases. Here are some:
<ul>
<li>Current version always downloads selected linked images. This works good with images hosted on external sites. 
If your posts link to images hosted in your media library, you will end up with two images.</li>
<li>A more user friendly name should be used for images added to the library (e.g. Thumbnail for post …)</li>
<li>Somewhere, a button to reset the “previously scanned” status of all posts should be provided (on current version, 
you cas set or reset it for single posts in the edit page.</li>
<li>“previously scanned” status should probably be reset (or at least you should be asked to) when publishing a 
updating an existing post</li>
</ul>
More ideas are wellcome.

Automatic Thumbnail Generator

Automated on-demand generation of posts thumbnails.

cyrillbolliger

Easily control the visibility of the featured image on singular posts and pages–while keeping it visible in archive pages, query loops, and other list views. This plugin provides a simple checkbox option within the post editor, allowing you to enable or disable the display of the featured image on individual posts and pages.
<h4>Key Features</h4>
<ul>
<li>Show or hide the featured image on singular pages and posts.</li>
<li>Seamlessly integrates with the WordPress post editor.</li>
<li>Simple checkbox toggle—no technical knowledge needed.</li>
<li>Compatible with most themes.</li>
<li>Supports WooCommerce product pages.</li>
<li>Lightweight and optimized for performance.</li>
<li>100% free—no ads, no upsells, no premium versions!</li>
</ul>
Perfect for bloggers, content creators, and developers who want precise control over the visibility of featured images on a per-post basis.
<h4>Important Notice</h4>
If your theme uses a custom method to load the featured image (such as the Twenty Seventeen theme), this plugin may not work. To ensure compatibility, use standard WordPress functions like <code>get_the_post_thumbnail()</code>, <code>wp_get_attachment_image()</code>, or the <a href="https://wordpress.org/support/article/post-featured-image-block/" rel="ugc">Post Featured Image</a> block.
Additionally, by default, this plugin only hides the featured image when it is loaded inside the loop. If your theme loads it outside the loop check out the first FAQ entry for a solution.

Conditionally display featured image on singular posts and pages

It works!

I was able to get this working on Genesis Press themes by adding the following code.

function wp234_featured_post_image($v) {
 global $post;

	if ( is_singular( 'post' ) && get_post_meta( $post->ID, 'cybocfi_hide_featured_image', true ) ) {
 return ' ';
	}

 return $v;
}
add_filter( 'genesis_pre_get_image', 'wp234_featured_post_image', 8 );

Works great!

Easily control whether the featured image appears in the single post or page view (doesn't hide it in archive/list view).

Automatic Thumbnail Generator vs Conditionally display featured image on singular posts and pages