Alex Mills

Regenerate Thumbnails allows you to regenerate all thumbnail sizes for one or more images that have been uploaded to your Media Library.
This is useful for situations such as:
<ul>
<li>A new thumbnail size has been added and you want past uploads to have a thumbnail in that size.</li>
<li>You’ve changed the dimensions of an existing thumbnail size, for example via Settings â Media.</li>
<li>You’ve switched to a new WordPress theme that uses featured images of a different size.</li>
</ul>
It also offers the ability to delete old, unused thumbnails in order to free up server space.
<h4>In Memory of Alex Mills</h4>
In February 2019 Alex Mills, the author of this plugin, <a href="https://alex.blog/2019/02/27/from-alexs-family/" rel="nofollow ugc">passed away</a>. He leaves behind a number of plugins which will be maintained by Automattic and members of the WordPress community. If this plugin is useful to you please consider donating to the Oregon Health and Science University. You can find more information <a href="https://alex.blog/2019/03/13/in-memory-of-alex-donation-link-update/" rel="nofollow ugc">here</a>.
<h4>Alternatives</h4>
WP-CLI
If you have command line access to your server, I highly recommend using <a href="https://wp-cli.org/" rel="nofollow ugc">WP-CLI</a> instead of this plugin as it’s faster (no HTTP requests overhead) and can be run inside of a <code>screen</code> for those with many thumbnails. For details, see the documentation of its <a href="https://developer.wordpress.org/cli/commands/media/regenerate/" rel="nofollow ugc"><code>media regenerate</code> command</a>.
Jetpack’s Photon Module
<a href="https://jetpack.com/" rel="nofollow ugc">Jetpack</a> is a plugin by Automattic, makers of WordPress.com. It gives your self-hosted WordPress site some of the functionality that is available to WordPress.com-hosted sites.
<a href="https://jetpack.com/support/photon/" rel="nofollow ugc">The Photon module</a> makes the images on your site be served from WordPress.com’s global content delivery network (CDN) which should speed up the loading of images. Importantly though it can create thumbnails on the fly which means you’ll never need to use this plugin.
I personally use Photon on my own website.
Disclaimer: I work for Automattic but I would recommend Photon even if I didn’t.
<h4>Need Help? Found A Bug? Want To Contribute Code?</h4>
Support for this plugin is provided via the <a href="https://wordpress.org/support/plugin/regenerate-thumbnails" rel="ugc">WordPress.org forums</a>.
The source code for this plugin is available on <a href="https://github.com/automattic/regenerate-thumbnails" rel="nofollow ugc">GitHub</a>.

Regenerate Thumbnails

I'm a longtime WP dev and if you need to regenerate thumbnails its the way to go.

I appreciate the fact its free, this is what makes WP great

5 Stars


I can't even remember for how long I've been using this plugin from time to time... Miss you Alex... I'm sure you're smiling from up above 3> 


Thanks, Alex...

Regenerate the thumbnails for one or more of your image uploads. Useful when changing their sizes or your theme.

Jess

Thumbnail Updater does exactly what it’s name implies: it updates thumbnails. WordPress 2.9 introduced support for 
post thumbnails but one problem is that the support is not backwards compatible—meaning that if you downloaded or created 
a theme that supports post thumbnails, any images uploaded prior to the activation of that theme will not have the new thumbnail size.

Thumbnail Updater

A plugin for updating your thumbnails whenever a new thumbnail size is added with add_image_size()

Regenerate Thumbnails vs Thumbnail Updater