Multiple images underneath featured image

Hi everyone,

I’m having trouble with the product pages on Mystile.
I want to get multiple product images underneath the featured image. Like this:

https://www.asymmetree.nl/shop/zodiac-series-aquariu/

But I only manage to do so, by ‘setting featured image’ and then upload multiple. After that, you can not check multiple images, only one, but somehow it does add the other ones.

One problem with this method is that I can not change the images, they do not show up anywhere in my admin panel, and the featured images only refers to the one.

The other problem is that I want to use the same image on several products. If I need to do it like explained above, I need to upload this image again for each product.

And finally, a third problem, these images don’t seem to be linked. This resulted in me once removing all unlinked images, and all the small images underneath my featured image were gone.

I am probably overlooking something, can someone give me some help?

Thanks!