Like Wikipedia and del.icio.us, Flickr too joins a growing list of websites that are ranked very high in search engines but do not pass any link juice to external websites.
Flickr has recently enabled the nofollow tag for all HTML links placed in photo descriptions and comments – that means Flickr will not pass any PageRank benefits to external sites. See example.
And though Flickr has nofollowed all external links, they still want external websites (who post Flickr photos) to link to the original photo page without using the nofollow tag (check the HTML code generated by Flickr when you want to link to some photo).
Surprisingly, the nofollow tag is not applied if the URL in the photo description points to another page on Flickr. And external links in descriptions of Flickr Sets and Photo Pools continue to pass PageRank as noted by Jeff Muendel but am sure that will change very soon.