When you install the WooGallery plugin on your site, it allows you to add additional gallery images for all of your product variations. Follow the steps below to do it: 

1) Go to the Edit Product page of a variable product.

2) From the Product Data section, click on Variations. 

3) Select a product variation, and you will find the Variation Image Gallery. 

4) Now, add as many images as you want.