You can perform two different bulk actions for images at the Variant level.
1Change the default image for multiple variants
To set the default image for multiple variants in bulk, select variant(s) within the product page, and from the Bulk Actions drop-down menu, select 'Change Default Image'.
Selecting 'Change Default Image' will bring you to the following screen:
To set the Default Image for the selected variant(s), click on any of the images and click on 'Attach Images'.
2Attach Images to multiple variants
To Attach Images to Multiple Variants, within the product page, select the Variant(s) and from the Bulk Actions drop-down menu, select 'Attach Image(s)'.
Next, you will see the following screen:
Select the image(s) you wish to attach to the selected variant(s) and click on 'Attach Images' to add the images to the selected variant(s).