To manage variant images go to Inventory > Variants > Variant name, then select the Images tab. From here you can view currently attached images, or choose to attach more images to the variant.
When uploading images to your variants, note the image will be available to attach to all other variants under that particular product.
From the main image screen, you can also choose to detach images from the current variant. To detach images, click to select the image (also works with multiple selections), then click the Detach Images option.
Un-assigning an image from the variant does not delete it from the product. It just removes it from the variant.
Once you've completed making your changes, click the green Save Changes button.