Image regeneration in WordPress is a very useful feature. This is not included in the WordPress core but can be achieved using plugins. One of them is Image Regenerate & Select Crop, a free WordPress plugin that manages this in a very user-friendly way. It has no limitation in terms of the number of images to regenerate/crop and it includes several optimizations and a lot of settings.
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Please see below a few mentions of my plugin on other websites, from people that tested, use, or recommend my plugin. These are not sponsored messages, I just found these on the web, and I thought to share these with you.
...it’s quite practical for regenerating thumbnails after creating custom images sizes or changing themes. With all the available tools inside the plugin, you can take care of all the images on your site, even for responsive sizes.
If you are annoyed at the fact that WordPress generates fifty thousand different thumbnails of every single image you upload, you can use this to go through and say for what post type what image size you need and delete the ones you don't want... this one is something that's easier to use without technical knowledge and it would just clear out some of the clutter in the media library...
This will probably be a WordPress core feature before too long, but until then if you need to set the crop for your pictures and thumbnails, this is our favorite plugin to do it. There are other options that seem equal or better in terms of installs and reviews, but this is the one we have been using.
You can play around with the image sizes to get the best result for your store or WordPress Theme. But be aware that WordPress won’t automatically update the image sizes. You will need an image regeneration plugin to change all the image sizes... you can use a more advanced plugin that will let you crop uploaded images too.