If your theme has the functionality to show the main image instead of the selected variant on load you can do that with these settings.
- Open Variant Option Product Options
- Go to the Settings
- Go to Show First Variant’s Image on Load
- Select Off
- Click Save
If your theme doesn’t have the functional to show the main image at load you will need to talk to your theme developer or Shopify for assistance with this. https://help.shopify.com/en/questions#/contact