Feature image for product with variations

Feature image for product with variations
Dear,
For this product I try to add a feature image but it never allow me to save it.
So I choose it, save it, but then it disappears.
This is a product with variations.
So I put a photo for each variations but when I list the product they appear without foto.
I tried to add the feature image but it doesn't allow me to do it as said above.

Feel free to see the shop online
http://www.pixshopping.com/countrytown/

I grant you support access...

Thanks.
Paddy