Collage is considered fair use, and therefore not subject to copyright law.
The doctrine of fair use protects educational and scholarly purposes such as news reporting, literary criticism, and libraries. Artistic uses are not explicitly protected by fair use, and commercial uses are explicitly not protected.

My collage can freely use copyrighted material, as long as I use no more than 5% / 10% / a small amount of the original work.
The doctrine of "de minimis" theoretically protects minimal copying, but it's vague and difficult to pin down.

I can use copyrighted material in my collage, as long as I don't reproduce it / make only a few copies / give it away / donate the profits to charity.
Collage is considered a derivative work, sometimes called a "new version" of the original work. The US Copyright Office says that "Only the owner of copyright in a work has the right to prepare, or to authorize someone else to create, a new version of that work." If you are infringing on someone's copyright, the lack of blatantly commercial purposes may decrease your risk of damages in the case of a lawsuit. But infringement is still infringement.

If something is published without a copyright notice / on display in a public place / posted on the Internet, it's not copyrighted, and therefore free for the taking.
Since 1989, anything that can be copyrighted is automatically copyrighted, as soon as it is created in a tangible form. (Yes, "tangible" includes the Internet.).

My collage is a parody, therefore it is protected.
According to the Copyright Office, fair use does protect "use in a parody of some of the content of the work parodied." But there are two things to remember. First, the use must be a legitimate parody of the prior work. For example, using images of Barbie to parody Barbie is a fair use. Using images of Barbie to make a statement about society in general, where another doll would do just as well, is not. Second, parody is a form of cultural criticism, but not every cultural criticism is a parody. Is the usage truly a parody? Would a court agree? Think hard about that before relying on parody to protect your work.

I can protect my intellectual property rights by mailing copies of my creative works to myself and saving the unopened, postmarked envelopes.
This pernicious myth is called "poor man's copyright" and it has no benefit to the copyright holder. None whatsoever. It doesn't matter if you keep the envelopes in a safe deposit box, give them to your lawyer, or bury them in a time capsule. A postmarked envelope has no value in protecting your intellectual property. Copyright applies automatically to your work, but if you are concerned about infringement against you, it's worth the small fee to register your work with the Copyright Office.

This legal mumbo jumbo is all well and good, but it only really matters to the big boys. No one's going to sue me.
Many artists assume that if they have no substantial assets, then they have nothing to lose, so no one would bother to sue them. It's true that you can't pay money you don't have. But you could be forced to cease publication, shut down your web site, or even to destroy all copies of art which includes copyright infringement.

Some artists get away with ignoring copyright, how can I do it?
If you think I'm going to publicly advise artists on how to circumvent the law, you're crazier than I am!