If your thumbnails are not correctly cropped, you can regenerate them by following these steps:
1. Go to Plugins > Add New
2. Search for “Regenerate Thumbnails” (created by Viper007Bond)
3. Install and activate that plugin.
4. Go to Tools > Regen. Thumbnails
5. Click “Regenerate All Thumbnails” button and let the regeneration process finish to 100%.