Flickr is one of those sites I knew about but never really used. I can see its usefulness for sharing your own photos and for sourcing creative commons images, but as with everywhere on the web you can never be 100% certain that the people that posted the images were entitled to. For example, I know that Trinity College are very careful about images of the Book of Kells, charge per use and would never allow them to be posted to the internet under a creative commons license, and yet here it is. I have huge problems with the idea of restricting access to images of documents that are hundreds of years old, and I believe if the user was based in the US he would be allowed to reuse it although Irish people (to whom the book really belongs!) are not. The joys of copyright and the internet! Of course, if someone claims rights of an image on this site I'll just take it down.
One of the interesting things about Flickr is the fact that people use different images to create new ones. The image above, for example, which I used in this post, was created by sammydavisdog using three other images. Very artistic, I think, and a nice way to add a personal touch rather than just reusing generic images.