Using other's copyrighted works without permission is not permitted.
Copying an entire work and then selling copies on the street.
Copying the entire work and then making some other commercial use of it.
For example: a photo on someone else's website.
If you're not *directly* commenting on that image in a noncommercial fashion, then you should get permission.
Difficult because you can't always get a hold of the photographer.
A quote you read on a website -- OK - with proper attribution. (Fair use.) Otherwise plagiarism.
Good idea to always try ask and receive permission (in writing).
Example: So don't get a Salon account, copy all of the articles, and then charge your friends a buck a month to read them on your site. You will get into trouble.
If you're not sure about how you're using a work, *call a lawyer*.
The only way to be absolutely safe is to get permission for the work you use, or create your own.