There is no requirement to post credits for anything. Lots of people feel that the exchange of money is thanks enough. However, when it comes to free stuff, many, if not most feel that a thank you should be given to the creator of those because it's the only credit/acknowledgment they get. But again, it's not mandatory either....just a nice gesture. I just did an image that had different head and body morphs, a different texture, different eyes, different eye glint, different lips. By the time I put it all together and tweaked things myself, I had lost track of it all. And that was for a single image scene. I can only surmise that it gets much worse with some of the really complex images that I've seen in the gallery.

"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