According to that article, it sounds like it mostly depends on how popular your website is, and unfortunately the court didn't offer any suggestions for what constitutes sufficient "economic significance", other than the bizarre claim that postcards don't meet that threshold (although I'm sure that far more money has been made off of Swedish FoP postcards than images from Offentligkonst.se). In the absence of any guidelines, will people want to take the risk of even posting individual images? I'm sure BUS will start threatening to sue anyone who doesn't pay licensing fees regardless, as the lack of clarity certainly plays to their advantage.