Thats an interesting article, which I do not agree with.
And as far as legality goes, its debatable. It tends to be a question of whether or not you sell anything. If you aren't selling anything and it is not possible to demonstrate that the users of borrowed art are causing harm to the creator/owner then there is no problem.
Good luck establishing "harm".
As far as I am concerned - if the big companies want to crack down, then let them. But don't make it easy for them to win a case. Don't sell the stuff. Don't scream to the heavens that you have the stuff. Modify the stuff and make it your own if you can.
Make it hard for the bastards to ruin our fun.