At some point, especially if you change a WordPress theme, you might decide to resize your thumbnails. You may choose to make them bigger, or simply change the proportions because you don’t like how they are cropped. It’s easy to do 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.