If your thumbnails are not correctly cropped, you can regenerate them by following these steps:
- Go to Plugins > Add New
- Search for Regenerate Thumbnails (created by Viper007Bond)
- Install and activate that plugin
- Go to Tools > Regen. Thumbnails
- Click Regenerate All Thumbnails button and let the regeneration process finish to 100%
[EDIT] On WooCommerce 3.3 and newer you will no longer to have to use the “Regenerate Thumbnails” plugin when changing images size as this is done automatically in the background now by WooCommerce.