There are many artists that appropriate other images as elements of their work. You may also want to use an image to prove a point in a paper or put it on your website. However, it's important to be careful and be aware of the laws governing image rights. Here are some resources that may help you figure out whether you can use an image or not:
You may have seen, on Flickr or elsewhere, captions that say that an image is protected by the Creative Commons license. Creative Commons is a type of copyright or licensing agreement between the creator of an image and everybody else on the Internet. If you look closely, the Creative Commons license that goes with an image will tell you exactly what you can and can't do with an image. There are four basic qualifications:
These four factors can be combined in various ways. You can look at the official website of the Creative Commons to find out (much) more about this.