Nico

<a href="http://nextgenthemes.com/plugins/regenerate-thumbnails-reminder/" rel="nofollow ugc">Plugin Page</a>
A small reminder in your admin area every time a image setting was changed (in media options, through a theme or plugin)
You most likely want to regenerate the thumbnails after this plugin reminds you to do so, if not already done I suggest you install the most used plugin Regenerate Thumbnails for this. This plugin will offer a redirection to <a href="https://wordpress.org/plugins/regenerate-thumbnails/" rel="ugc">Regenerate Thumbnails</a> in the admins reminder notice, but you can use any other plugin/method you prefer to regenerate thumbnails, just click dismiss when the reminder shows up and then manually navigate to your preferred regenerate plugin.

Regenerate Thumbnails Reminder

Checks if your image sizes have changed or if there was a new one added, if so it reminds you to go regenerate them.

Mohammadreza Maghsoudipour

Are you tired of WordPress creating dozens of unused thumbnail sizes for every image you upload?
By default, WordPress and many themes register multiple image sizes and generate them for every single upload, filling your server with thousands of files you will never use. This wastes disk space and makes managing your media library difficult.
Smart Thumbnails GW solves this problem.
This plugin stops WordPress from creating unnecessary files and gives you precise control. Its intelligent logic generates custom sizes only for Featured Images when a post is saved. Images uploaded into your post content won’t create any extra thumbnails, saving you valuable server space.
Control exactly which sizes are created for each post type (posts, pages, products, etc.), clean up unused images automatically, and regenerate thumbnails only when you need to.
<h3>Source Code</h3>
The full source code, including the unminified JavaScript files, is included in the plugin directory under <code>assets/js/</code>.

Smart Thumbnails GW

An intelligent image management plugin that registers custom sizes for each post type and regenerates them only when needed for featured images.

Regenerate Thumbnails Reminder vs Smart Thumbnails GW