I won't labor the details of copyright law here, but the history of it is both fascinating and ironic, if you get curious enough to look it up. Sadly, it is both poorly understood, maladapted to modern technology and largely ignored by the public at large.
In my own experience, I have found the letter of the law and the practice of the law to be two very different things. The letter of the law has been labored to death here. The practice, however, is what really happens when the rubber meets the information highway. In real life, I have observed that:
1) Most people will not resell your stuff without permission.
2) They will not hesitate to give it away, however. If you don't want it circulating on the web, don't post it.
3) If you DO want it circulating on the web, you won't be able to give it away.
4) Victory goes to the one who can afford the most lawyers.
5) The general public confuses copyright and the right to publicity all the time. Copyright is international law. The right to publicity (the ability to control the use of your own image or name) is state law, and it varies from state to state. It does not exist at all in many states.
6) You cannot copyright an idea, but you can sued for anything.The likelihood of your getting in trouble for some sort of IP infringement is directly proportional to how much money you are making.
7) The estates of famous creators guard their IP far more jealously than the creators themselves ever would have. That's because the motive of creators is to share their creations with the world, and the motive of their heirs is to wring every last cent from them.

The same can be said for organizations like stock houses, record labels and rights organizations like the RIAA or MPAA. They are far more concerned about controlling distribution than the people who actually labored to create what they are selling. They gleefully rip off content creators themselves.
9) Watermark your photos/art/movies. It won't stop someone from swiping it, but it will at least serve as an ad if they do. (Some particularly lame people will delete or crop your watermark. You can make life harder for them by overlapping a complex and irregular background object with your watermark.)
10) A good rule of thumb that is far easier to remember than the vagaries of IP law is simply this: "Don't be a dick". If you want to use someone's artwork/photo/song/whatever...just ask them first. Likewise, if you want to take a picture of someone, ask them first. (Yes, in a public place, you can take their picture, anyway, but would it hurt to ask? You might even get a better picture, or make a friend.) If you DO use something, give them visible credit. It's called civility and it is a wonderful alternative to litigation.