From your Shopify admin, go to Products > All products.
Click the name of the product that you want to edit.
In the Variants section, click the image icon to assign an image to that variant.
Select a product image or click Add image to upload a new image.
Let me know whether it works.
The default has to be the first variant option in the list. The only way I know of to change the default variant without custom coding is to reorder your variants, see here: