Set all images name of a product to the same field in other table separated by comma in MySQL
You can use GROUP_CONCAT().

UPDATE importer 
JOIN (
    SELECT product_reference, GROUP_CONCAT(name) AS name
    FROM Product_images 
    GROUP BY product_reference
) pi ON importer.sku= pi.product_reference
SET importer.images = pi.name;


I should mention though, that your schema design may not be the best, depending on what you want to do. Read about normal forms, if you don't know what that is. A OLTP database usually is in at least 3rd normal form. Comma separated values violate the 1st normal form already.