When posting from Flickr, you are supposed to give a link back to the flickr image per your Flickr agreement:

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.

They do want you to post images from Flickr to elsewhere, like here on the forums, but they also want the traffic back to Flickr. It was great when they added the BBcode option to the 'share this' list.