Regenerate Thumbnails Plugin Free Download

Regenerate Thumbnails allows you to regenerate all thumbnail sizes for one or more images that have been uploaded to your Media Library.

This is useful for situations such as:

A new thumbnail size has been added and you want past uploads to have a thumbnail in that size.
You’ve changed the dimensions of an existing thumbnail size, for example via Settings → Media.
You’ve switched to a new WordPress theme that uses featured images of a different size.
It also offers the ability to delete old, unused thumbnails in order to free up server space.