I've done a little research into this.
Different countries have different rules. If you are convinced your product will never leave this country (not sure how you could do that), you could use the 70 year rule. But the UK rule is 100 years. If your product ended up in the UK, you could find yourself under copyright violation.
My example: I'm writing a story that uses Bram Stoker and Dracula as characters. Dracula is famously out of copyright because Stoker accidentally put it into the public domain (his family continues to fight it). However, Universal made a movie in 1931, and they own the copyright to the movie, or, their artistic interpretation of Dracula. As long as my Dracula only references the original literature, I'm fine, but if I reference something in the movie that is not in the novel, I could be in trouble.
Likewise, actors and celebrities make a living off their likeness, so even if they can't copyright their likeness, only they are allowed to profit from it or approve its use (like to endorse a product or a charity). However, public officials are exempt from this as I understand it. (There is a difference between public officials and public figures, too). You can make Obama stuff till you're blue in the face. Or Abe Lincoln stuff. In fact, you could conceivably create a stylized image of Abe Lincoln, and anyone who used your stylized version would be in violation of copyright. But you can't copyright a realistic image of Lincoln. The artist who created the tiny Keanu Reeves and Ryan Gosling would probably not get sued, but they might get a cease-and-desist letter, asking them not to sell them anymore. Silence is NOT consent.
Another example: There is a Van Gogh painting of a branch with blossoms on it. That is obviously public domain. However, one person took the image, took the one-inch border around the picture, and duplicated it twice, effectively making the image 4 inches larger in both directions with an "artistically modified" image being created in the process. If you happen to use their version accidentally instead of the original, they can sue you. (Look in the upper left hand corner, then you'll know what to look for in the rest of the picture.)
http://3.bp.blogspot.com/-RIebNjZvs2Q/UUC-1MsN9LI/AAAAAAAAPu U/XETd9dnZLKg/s1600/blossoming-almond-tree-by-vincent-van-go gh.jpg
The Statue of Liberty and the Eiffel Tower are in the public domain, and images of them are copyright free. However, the city of Paris got tired of not making any money from the thousands of photos taken of the tower every year. They hired an artist to create a light display that hangs on the tower. Photos of the tower by day are fine. Photos of the tower at night are actually photos of the lights on the tower, and that is copyrighted. Bastards.
Back to Dracula and Stoker. Drac is public domain everywhere, and Stoker's likeness was public domain once it was 70 years old - but, because I didn't want to get in trouble in the UK, I had to wait until the 100th anniversary of his death, which was April 20, 2012. Just to be safe. I also had to leave out another author and his character from my story because, while the story is public domain here in the US, it's still under copyright for a few years in the UK. I could wait it out or do without the author/character, so I dropped him.