I'm not a lawyer, I'm an artist. I have done some of the things you are taking about. As far as I know this is how it works:
If you make something (art of any kind) that is a one time piece, for yourself or for a gallery then it is fine to use "most" images you find in magazines, books, postcards, so forth... as long as you are not using someone elses fame to get an advantage, or to take away from their fame.
If you are planning to sell or mass produce your work, then you MUST get premission from EVERY copywrited picture you want to use. Even if you are only using small, small parts of a picture.
There are books that explain copyright laws (in eazy to understand) terms for artists, that you can find at arts and crafts stores, of course bookstores, and even places like fabric stores... for artist that use copywrited images for quilts and such...
And of course... alot of people use images all the time without getting caught. I'm happy to see that there are people like you wanting to do the right thing from the start.
Xlldpo
(Artist, NOT a Lawyer)