One of the questions that came up during the New Bloggers series was - is it OK to use photos from Flickr?
Basically, what it says is that you should be looking for photos listed under the Creative Commons agreement in the "Attribution" category.
If you find a photo that is copywrited, you need written permission to use it. Under the Creative Commons agreement, you are allowed to use photos without written permission, as long as you credit the photographer and give a link back to the source. In this case, their Flickr page.
There are six separate categories under the Creative Commons agreement, so you'll want to read the whole article. She also tells you how to find photos that are offered in those categories.
Hope this helps all of you looking for images!