The use of other's printed and copyrighted photos or art is a huge problem. It may be tempting to use something from a magazine or advertisement that inspires you, and you're allowed, if it is just for your own viewing and practice. It is, however, illegal to sell, print, use for display in any kind of public showing, or give the impression that it is your art in any way, without express written permission from the original creator of that source.
This doesn't mean that you can't use the idea as inspiration for shooting your own reference photograph-when you do shoot your own reference, it is automatically protected from other's using it as well.
Beyond creating your own reference material, it has been getting easier to find royalty free material to use on the Internet. Additionally, other artwork and photography that the copyright has expired on and that are now considered a part of the public domain can be found through the Library of Congress.
However, as an artist, I recommend that you make the effort to have all of your artwork and material officially copyrighted through the Library of Congress-this gives you additional protection and the rights to claim damages, claim profits made through the use and infringement of your artwork, as well as any court costs incurred in pursuing your case.
I encourage everyone to make sure they have gotten the necessary permission or made sure it is their own reference before doing something for any kind of display. This is a good habit to get into, because a drawing that you thought was just for practice, or for our own use, may turn out to be something that you are particularly proud of and would like to publicly use, publish, or even sell.
As an artist, I see that many people around the world have used my artwork without permission, taken my name off of it and/or the copyright notice-then claimed they had no idea who did the work, or they claim that it was in the public domain. It's frustrating for the artist, to say the least.
Looking at catalogs, books, magazines and other's artwork can be great for inspiration and a terrific source for ideas. There are a set number of changes that you can make to consider it your own artwork, but this is both subjective and tricky-therefore I feel more comfortable creating my own reference. That way, every single facet of my art is my creation and I do not have any worries. It's always better to be safe that sorry!
Darrel Tank has helped thousands of students learn to draw realistically with his unique 5-Pencil Method.
Get your 3 hours of free drawing tutorials at http://www.fivepencilmethod.com/free-drawing-tutorials/.