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Note: This functionality can be found in: https://github.com/markguinn/silverstripe-shop-extendedimages

Decorate Product, and add many_many AdditionalImages. Update CMS fields, so images can be added.

[mysite]/code/MultipleProductImages.php

class MultipleProductImages extends DataExtension{

    private static $many_many = array(
        'AdditionalImages' => 'Image'
    );

    function updateCMSFields(FieldList $fields){
        $fields->addFieldToTab('Root.Images',
            new GridField(
                'AdditionalImages',
                'Images',
                $this->owner->AdditionalImages()
            )
        );
    }

}

Add the extension in _config/config.yml

Product:
    extensions:
        - MultipleProductImages

[mysite]/templates/Includes/AdditionalImages.ss

<% if AdditionalImages %>
    <% loop AdditionalImages %>
        $Me
    <% end_loop %>
<% end_if %>

Add <% include AdditionalImages %> somewhere in your Product.ss template.