hot to make same pictures gallery for all variants

i am using woocommerce (ver. 3.3.3 newest)
i am building a woocommerce shop for a client.
all of the product(for now) are variables-product which differs by size.
that means that all variants of the same product should have the same gallery pictures.
but when i choose a gallery pictures for the product it appears in the product page only before the costumer choose a size.
when the costumer enter to the product page he can see the thumbnails as i defined but after he choose a size it disappears.
so i need to add the same gallery to any variation for this to not happening.
is there a better way to define that any variation will have the main gallery?
plugin, programming, settings?

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