I think giving credit should be based on what the individual, by no means should anyone feel obligated to do so whether the content is free or purchased.

However if you do use someone else’s content and they request credit or even and email with of the rendered picture this would only be fair and should be respected.

I have seen some great art created with free and purchased content and rarely do I read who all the contributors are. I like to see good art no mater if its all original or not.