The plugin allows you to regenerate the thumbnails after changing their size.

Regenerate Thumbnails – regenerate thumbnails in WordPress blog

This solution allows WordPress to regenerate thumbnails for your images and attachments. It is very inconvenient to manually change a large number of miniatures, especially if you have previously changed their sizes through CMS menu – Settings – Media after downloading images or, for example, changed the subject.
You also have the opportunity to generate thumbnails for all images are going to load an individual photo or special snapshot in large numbers.

Regenerate Thumbnails – regenerate thumbnails in WordPress blog

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