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.
You can either regenerate the thumbnails for all image uploads, individual image uploads, or specific multiple image uploads.
See the Screenshots Tab for more details.
Installation and Usage Method:
1. Go to your admin area and select Plugins -> Add new from the menu.
2. Search for ‘Regenerate Thumbnails’.
3. Click install.
4. Click activate.
5. Go to Tools > Regen. Thumbnails
6. Click “Regenerate All Thumbnails” button and let the process to finish till it reaches 100 percent.