Sorry to hear of this....BUT...it has happened many many times here before...and it will again!!!
Sadly, Renderosity will do nothing about it.  They  have stated it is up to the artist to enforce copyright violations...which is true...BUT, there are things that Renderosity can do to help stop image theft.
I have offered those suggestions over the past year....none have been implemented.

@e-brink....I too have started to watermark all my images...and yes, I too have had people complaining.....If they don't like it that's too bad!  Once they have had their own images stolen....I am sure they will change their mind.

Good luck to all in getting your images removed from the site in question...and remember, you are only seeing the tip of the iceberg....there are possibly many of your images that may get printed and sold in street markets...those, you will never find....

In youth, we learn....with age, we understand.