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.

cubecolour

Add the [zoom] shortcode to a page, post or custom post-type post. If the post has a featured image it will displayed on the page, and when you move the cursor over the image, the image will be magnified with the image viewport remains constrained to the dimensions of the original image.
on a mobile/touch device the image will also be zoomable, but as there is no hover state this is achieved by means of a pinch & zoom action.
On my site you can see a <a href="https://cubecolour.co.uk/featured-image-zoom/" title="demo of Featured Image Zoom" rel="nofollow ugc">demo</a> of the plugin.
<h4>Usage:</h4>
Ensure that your post/page has a featured image defined.
Use a [zoom] shortcode to your page.
Some optional shortcode parameters can also be used.
=size= 
The large image is used by default for the unzoomed image. Use the size parameter to use a different defined image size.
eg ‘[zoom size=thumbnail]’
=zoomsize= 
The full image is used for the zoomed overlay. This can be changed using the zoomsize shortcode parameter.
eg ‘[zoom zoomsize=large]’
The image size definitions used for the the unzoomed and zoomed images should have the same aspect ratio and cropping.
=zoomin= 
This can be used to tweak the level of zoom. The value must be a positive integer. The default is 6 so if the zoom level results in too much magnification on hover, try a lower value.
eg ‘[zoom zoomin=2]’

Featured Image Zoom

Add a [zoom] shortcode to display a zoomable featured image.

Automatic Thumbnail Generator vs Featured Image Zoom