Theme Egg

Generate Thumbnail allows you to generate all thumbnails on your site. There are already some plugins available for this, but they have one thing in common: All thumbnails are regenerate in a single step. This works fine when you don’t have that many photos on your site. When you have a lot of full-size photos, the script on the server side takes a long time to run. Unfortunately the time a script is allowed to run is limited, which sets an upper limit to the number of thumbnails you can regenerate. This number depends on the server configuration and the computing power your server has available. When you get over this limit, you won’t be able to generate your thumbnails.
Why would you want to generate your thumbnails? WordPress allows you to change the size of thumbnails. This way, you can make the size of thumbnails fit the design of your website. When you change the size to fit for a new theme, all future photos you are going to upload will have this new size. Your old thumbnails won’t be resized. That’s where this plugin comes into action. After changing the image sizes, you can generate all thumbnails. But instead of telling the server to recreate all thumbnails at once, they are generate one after another. Generateing thumbnails for one photo won’t take all too long, so you won’t run into any script timeouts. Note that you still have to wait until all thumbnails have been rebuilt. If you close the page before the task is completed, you have to start all over again.
You can also select the thumbnail sizes you want to generate, so that you don’t need to recreate all images if you’ve just changed one thumbnail-size. You can also choose to only generate post thumbnails (featured images).
This plugin requires JavaScript to be enabled.

Generate Thumbnail

Generate Thumbnail allows you to generate all thumbnails at once without script timeouts on your server.

Jordy Meow

When you insert images in WordPress, it’s always a specific image size. It can be an image size such as Medium, Large or something else set by your theme or another plugin. When you update your media sizes settings (or switch theme), you need to regenerate those images (with Regenerate Thumbnails, for example). But what about your posts content? They will be still using the old images, which aren’t actually part of the new metadata, and little by little your WordPress will become more and more broken. This plugin fixes this by regenerating the HTML for your thumbnails / images in your content and keeps everything clean. Once this is done, it’s a good idea to run a tool like Media Cleaner (https://wordpress.org/plugins/media-cleaner/) on your install to detect the useless files.
Be careful!. Before using this plugin, backup your database! It doesn’t modify your files, only database.

Regenerate Thumbnails HTML

Works well on my two plugins after switching to a new theme. Regenerate Thumbnails (after set up the new sizes) + Regenerate Thumbnails HTML (on all posts, for 2 or 3 sizes) + Media Cleaner (to delete the useless files). In one hour, the new installs are neat and clean.

It works well

Thank you!
After change WP media sizes for new theme we must use regenerate thumbnail plug-in. But I want apply new sizes in old post! After that I can clear old images!

Good work with "view-broken-links" plug-in.

Gold in ocean of wp problems

Update the HTML of the images contained in the post from one image size to another. Useful when switching between sizes and themes.

Generate Thumbnail vs Regenerate Thumbnails HTML