Patt,
Let me start by saying I'm not a lawyer, let alone a lawyer skilled in copyright infringement. Although you might find some really obvious copying and violation examples, there are many more that are less obvious.
Let me recommend an excellent START to learning about copyrights, Tad Crawford's "Legal Guide for Visual Artists". (Get this or other books through the :
http://www.portraitartist.com/bookstore/business.htm.)
My belief is that if you do something that is for your own home, and has no possible implication to you for monetary value (yep, maybe even in your portfolio), it's pretty unlikely that you will be sued. Investigate for yourself the implications of clearly copyrighted material. You could find yourself liable for 6 figures in damages. Why in the world would you do this????
There's a whole world out there of original subjects to paint. I just say again, why would you want to find out how close you are to the line of a copyright infringement? We have all heard of anecdotes where the Disney Corporation has filed suit against nursery schools who have Disney murals in their playrooms.
And yes, I am aware that there is an enormous industry of copiers who go on and on selling copies. So?
If you feel compelled to paint copies of someone else's work (and BTW, be aware that, in the example of using an image from National Geographic, you would need releases not only from the magazine but from the photographer as well - there are copyrights, and underlying copyrights), and if the copying would result in your monetary gain, you should talk to a lawyer who either understands copyright law, or who has a good malpractice policy in place.
Happy reading! This is such an interesting subject!