I now know, that it is possible to show only the pictures of a particular product variant in your store instead of all pictures of all variants. This is possible with the apps like "Variant Image Wizard".
However, I wonder how those apps work.. Do they adjust your shopify theme code and use alt tags of the pictures to show the right images? I ask this question because I would like to sync my products with marketplaces like amazon. If it really works like that, this would basically mean it is impossible to tell amazon which pictures belong to which product variant..