Public domain belongs to the public. You can do anything you want with it -- even sell it. It belongs to EVERYONE. Once an item is explicitly placed in the public domain by the proper copyright holder, it cannot be rescinded. I mean, someone can't say "I place this super cool groovy texture I made in the public domain" and then come back and say "Nope, I change my mind, it's mine and you can't use it." Now, on the other hand, if I took Bob's texture, claimed it was mine and said I was putting it in the public domain, and you used it, you probably wouldn't be off the hook. If Bob found out and said "Hey, I never put that in the PD!", then AFAIK, it isn't in the PD. So you could get burned that way. As for the others, unless it says in the readme or whatever that you can do that, I would strongly suggest asking the freebie maker. It's really their call.