At some point, especially if you change a WordPress theme, your thumbnail sizes will change and they aren’t quite working with your new theme. Or you may choose to change them in your existing theme.
Now if you want to change the size or how a thumbnail is cropped, you can do that via your media settings. But remember, this only changes the size on future media that is uploaded.
The solution is a simple one. On your plugins page, search for the plugin “Regenerate Thumbnails”. It’s just another great plugin from Viper 007 Bond. Or you can upload it here at WordPress.org. What it basically does is regenerate all your previous media that you uploaded to your new thumbnail size. It’s that simple.
And you can either regenerate the thumbnails for all image uploads, individual image uploads, or specific multiple image uploads.
To regenerate all your thumbnails, you will find it under Tools > Regen. Thumbnails.
And if you want to do it to a specific image, you will find the option in your media library.
Or, if you want to choose several specific images in your library, you can use the bulk actions.