Assign images to product variants in bulk in Shopify Store

I am trying to assign images to product variants so that when an variant is selected the related images only show up.

I have tried using a couple of apps which work fine to do this but I have around 400 products and so need a way of doing this in bulk rather than manually one by one.

All 400 products are the same and have same variants and same image ordering but just each different product is just a different colour/pattern.

My products are all phone cases which all have the same variants on each product, the only difference between each product is the colour or pattern on the phone case. Each product variant has one main image which changes between each variant and then three more images which are the same for all variants.

If you feel you can help with this task please get in contact to discuss further.

Kind regards Sam