When you update your theme on WordPress, or change your theme it happens a problem: your old thumbnails sometimes will not match the current new design.
So what can you do to regenerate images on WordPress?
You just need to install this plugin.
This plugin will allow you to regenerate your thumbnails after changing the thumbnail sizes.
Regenerate Thumbnails allows you to regenerate the thumbnails for your image attachments. This is very handy if you’ve changed any of your thumbnail dimensions (via Settings -> Media) after previously uploading images or have changed to a theme with different featured post image dimensions.
After you have installed the plugin just go to Tools -> regenerate thumb and press Start