You may not freely use someone else's work simply because it has been posted on the Internet (a popular fallacy). Whether you find the material online or off, permission is generally needed to reproduce text, artwork, photos, and music. It is wise to operate under the assumption that all material is protected by either copyright or trademark law unless you have good reason to know that it is not.
As a general rule, any original work -- whether text, visual art, photos, or music -- is protected by copyright law, which means that you may not reproduce it without permission from the copyright owner.
If you do not cite the author, readers will think it is your work. This means you are claiming credit for someone else's work and is called plagiarism. It's illegal. If you publish someone else's work without the appropriate licence or permission you are committing copyright piracy.
Fortunately, a fair use exemption allows you to legally copy small amounts of someone else's work.