This feature is available from 2.1.8 version of Klevu Search extension.
- Login into Magento Admin panel
- Go to Stores → Configuration → Klevu → Search Configuration
- Select the appropriate store view from the Current Configuration Scope
- Under the Product Image Settings, select Yes for Use Configurable Product’s Image in Search
- Click on Save Config
- Once you have changed the settings, please synchronize product updates by following the steps specified at following link : Perform on-demand data synchronization