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.