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()

codersantosh

Acme Themes

Regenerate Thumbnails – Acme Fix Images is a handy WordPress plugin designed to resolve issues related to image thumbnail sizes. If you’ve recently adjusted image sizes in your Media Settings or encountered display problems due to improper thumbnail sizes, this plugin can help you ensure consistency and proper display of images across your website.
<ol>
<li>Log in to your WordPress admin panel.</li>
<li>Navigate to Appearance => Acme Fix Images.</li>
<li>Select the image sizes and options you wish to fix.</li>
<li>Click “Fix All Images” to regenerate thumbnails according to your chosen sizes.</li>
</ol>

Acme Fix Images – Regenerate Thumbnails

Fix image sizes after you have changed image sizes from Media Settings. Ensure your images display consistently across your website.

Thumbnail Updater vs Acme Fix Images – Regenerate Thumbnails