I also don't feel obligated to note everything I used in an image when I paid for it.  If a free item is promanently used, then it's polite to create the creator.  However, there again, it shouldn't be a requirement.  Presumably creators of free items do it because they enjoy giving (hopefully without strings attached).  If you really care about weather or not someone made, bought, or acquired for free some portion or an image, you can always ask them.  Personally, I only assume it was all made/modelled by the artist when they say so.  Seems to me that the folks doing that kind of work, have been noting it in their comments. 

Irene-