Hello Andrew,

We had the same issue for a client, we created a tool.

You can search the odoo store for :  Variant Photo Manager by Amofordesign
We did it for Odoo 17 , If you need  a different version let me know. 

Hi,

Method 1: Import File

    Export Product Variants

        Go to Inventory → Products → Products

        Apply filter: Product Type = Product Variants

        Click Export and include:

            External ID (required)

            Image field (or Image URL for web-hosted images)

    Prepare Your Import File

        Use the same image URL/path for all variants sharing an image

        Example: All "Blue Shirt" variants would reference

    Reimport the Updated CSV

        Go to Import Records in the Product list view

        Map the External ID and Image fields

        Validate the import

For a visual guide, refer to:

Odoo Product Image Import Tutorial

Method 2: Automated Assignment via Attribute Values (Technical)

For advanced users, create a custom rule to auto-assign images based on attributes:

    Enable Developer Mode (?debug=1)

    Navigate to Settings → Technical → Actions → Server Actions

    Create a new action with Python code like:

       for variant in records:

             if variant.attribute_value_id.name == "Blue":

                    variant.image_variant_1920 = base64.b64encode(

                     requests.get("https://your-image-url.jpg").content

                 )

Hope it helps

Did you see https://www.odoo.com/forum/help-1/how-to-import-images-from-the-import-wizard-263848