Pixelwelt

Image Sizes on Demand generates sizes of an image at the moment they are requested. It tries to generate the new size 
from the original image, if it exists. This means you don’t need to regnerate all your images if you add new iamge 
sizes.
<h3>Arbitrary section</h3>

Image Sizes on Demand

Creates the new size of an image, if it is requested and does not already exist.

Stefano Lissa

WordPress themes need thumbnails in different sizes and they even let the user to change those dimensions. 
But WordPress has a problem: it does not regenerates the thumbnails when new sizes are registered.
Thumbnails intercepts the request by themes or plugins to WordPress to get a specific thumbnail, generates it and caches it on disk. 
Efficiently, it produces a perfectly cropped image, with the right dimension avoiding unpleasant stretching.
Thumbnails does not modify your blog or your media library. When deactivated the blog returns to its old behavior.
The second important feature is the autoselection of featured image for post which are missing it. 
You can choose between the on the fly selection without persist it or, for better performances, to persist it once selected.
Read the <a href="https://www.satollo.net/plugins/thumbnails" rel="nofollow ugc">Thumbnails</a> official page for detailed information.
Theme developers can find instructions to use it (no code tied to Thumbnails needs to be written!).
Other plugins by Stefano Lissa:
<ul>
<li><a href="https://www.satollo.net/plugins/monitor" rel="nofollow ugc">Monitor</a></li>
<li><a href="https://www.satollo.net/plugins/hyper-cache" rel="nofollow ugc">Hyper Cache</a></li>
<li><a href="https://www.thenewsletterplugin.com" rel="nofollow ugc">Newsletter</a></li>
<li><a href="https://www.satollo.net/plugins/header-footer" rel="nofollow ugc">Header and Footer</a></li>
<li><a href="https://www.satollo.net/plugins/include-me" rel="nofollow ugc">Include Me</a></li>
<li><a href="https://www.satollo.net/plugins/ads-bbpress" rel="nofollow ugc">Ads for bbPress</a></li>
</ul>
<h3>Screen shots</h3>
No screenshots are available at this time.

Thumbnails and Featured Images

This plugin solves my problems with the WP thumbnails, thanks Stefano!

The one that solves my problem

I re-use my post thumbnail images in various ways, so I like to upload them full size, and I need the software to resize them as needed. This plugin does exactly that for me.

It performs its magic transparently and makes my pages a lot lighter. Highly recommended.


Amazing

I like this plugin's approach of generating thumbnails when asked.
I hope the plugin will be around for a long time.
Or if it were included in the core, it would cut down on wasted unused image files on servers around the world.

Now if only it would support WebP and responsive images, it would be perfect.

Thanks.

should be included in the core.

Autoselect the featured image and creates pixel perfect resizes on the fly without regenerate all the thumbnails.

Image Sizes on Demand vs Thumbnails and Featured Images