entity reference as attribute, is it possible in commerce v2?
Since product attributes are entities in CommerceV2, the following module should cover your needs => https://www.drupal.org/project/entity_browser

Some points to consider:


It is still in alpha
It is strongly supported at the moment by the community and especially the Drupal Media Team
It has more than 5K downloads.


There is also a recent presentation available from DrupalCon Dublin 2016 => https://www.youtube.com/watch?v=VXor-HZiYrs

Update: 
Following the documentation, you may also need to look into "Product Variations Fields", http://docs.drupalcommerce.org/v2/product/products.html.
This CV2 functionality seems to allow you to expose specific fields to the end-user for e.g. uploading images in order to modify/configure a product.

Good luck!