After the migration to WordPress, we recommend paying attention to your featured images. Sometimes the migrated thumbnails might display different size and figuration. It happens in cases when the size of images was not set on your current CMS before the migration.
You can fix this issue in the following way: please install a Regenerate Thumbnails plugin, which will generate new sizes for all the previously uploaded images.
Follow the instructions displayed on the plugin page and don’t forget to activate a plugin:
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. Navigate to Tools → Regenerate Thumbnails.