Hi Lech,
My name is Ardiane, a Guru here at Shopify!
For having control over the thumbnails shared on your Facebook page, you can have a look at this third party blog that I found that can help with that - How to: Force Facebook to update post thumbnail.
Also, you can have a look at our Shopify doc here that gives instructions on how to control the image that Facebook displays.
I hope that helps you out!
- Ardiane :)