from the Flickr Community Guidelines:
Quote:

Do link back to Flickr when you post your Flickr content elsewhere.
The Flickr service makes it possible to post content hosted on Flickr to outside web sites. However, pages on other web sites that display content hosted on flickr.com must provide a link from each photo or video back to its page on Flickr.
i just found this. i will often post a photo hosted on Flickr here on these forums. i pay for a professional membership to Flickr. i don't think i've ever linked back to the Flickr page. i could see this rule needing to apply if i post someone else's photo, but why should it apply to my own paid account?