Updated Terms of Use: On the 25th of October, 2004, we published a small modification to the Flickr Terms of Use. Formerly, posting photos on an external website required an explicit, visible link somewhere on your website back to Flickr. This particular term now reads:

“The Flickr service makes it possible to post images hosted on Flickr to outside websites. This use is accepted (and even encouraged!). However, pages on other websites which display images hosted on flickr.com must provide a link back to Flickr from each photo to its photo page on Flickr.”