So, I am not a lawyer, etc. The basics of copyright law, though, are fairly straightforward: If you don't own the image/text/song/whatever, you can't use it, unless there's an explicit license in place that lets you do what you want to do. There are some specific exceptions in place, like parody (which is about creating transformative work from pre-existing ones) and fair use, which has four main considerations:
- The purpose and character of the use, including whether such use is
of commercial nature or is for non-profit educational purposes
- The nature of the copyrighted work
- The amount and substantiality of the portion used in relation to the
copyrighted work as a whole
- The effect of the use upon the potential market for, or value of,
the copyrighted work
The important thing to note here is that you can't just say "no look, I'm not going to diminish your market or the value of your work, so it's totally fair use". Well, you can, but it'll be up to the courts to decide who is right, which means be prepared for some long, drawn-out legal battles. The best advice is to ask permission if a work is not clearly licensed by the original creator, and if it is clearly licensed (again, by the original creator, not just some random person on the internet), to abide by the strictures laid out in the license.