My suggestion?

The artists at Renderosity really have no clout with these people.

However, artists such as Royo et al, do.  (lots of Royo images at this particular site)  http://www.1zoom.ru     They have the legal representation and agents in place to enforce their copyrights.

My suggestion is to go through those images and identify every well known fantasy artist that has images in there and start sending out letters to them with links to their images.

Suggest teaming up with them as a group to get those sites shut down.  They have the pull where individual artists here do not.  And I think you stand a better chance of getting somewhere that way.

If these sites are suddenly hit with cease and desist letters from lawyers, they may start to get the idea that just taking art and displaying it on their own sites is not a kosher thing to do.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi