1) If you’re an employee of a company for whom you are generating images, music etc. the copyrights to that material belong to the company.
2) You do not have to place the copyright symbol with your name and “year created” near your published or printed materials- but if you do it’s easier to nail someone for infringement on your copyright if you go to court.
3) Pictures, compositions and other “works” whose copyright protection has expired (old stuff) or “works” that were never covered by copyright law (really old stuff) are not copyright protected and are considered in the “public domain” thus can be legally used by anyone.
4) Exceptions to copyright protection include newspapers using copyrighted materials without permission for reporting and teachers who make multiple copies of copyrighted materials for classroom distribution. There also seems to be some wiggle room with using copyrighted materials for portfolios, especially student portfolios.
5) If someone swipes your picture/song/video from the internet and uses it for their own purposes, it is a copyright infringement. (Btw, the same is true if you nick some else’s material for your own purposes.)