Really? Neither the word "instititution" nor "third party [website]" appear
in the text of the CC license, so on what exactly do you base this very
specific distinction just so narrowly fitting our behavior (no image
attribution within articles, only on the image description page reachable
upon clicking on the image), while not fitting anyone else doing exactly
the same? The license requires only that the credit "be implemented in any
reasonable manner". [Also note that the _text_ of our projects, while also
licensed under CC-BY-SA, is licensed in way that explicitly states that a
sufficient attribution is "[t]hrough hyperlink (where possible) or URL to
the page or pages that you are re-using (since each page has a history page
that lists all authors and editors)".]
1. CC-BY-SA is not defined by what Wikipedia is doing. CC-BY-SA is only
defined by its legal code.
2. If the licences states "You must" it means that "YOU Yourself need
to do this". And You yourself simply don't give "appropriate credit", if
you do not provide it yourself, but link to a third party website, you
don't have any control on and that maybe gone someday. Since one is not
liable for the content behind an external link, one cannot use it to
comply personal legal duties, on the other hand.
The attribution you give (Author and Licence) legally is the price you
pay for using this image. And if you do not give that attribution
yourself, you don't have any right to use that content.
3. You need to diffenciate between the practice within the wikimedia
projects and the one outside. No matter if the Wikipedia itself strictly
fullfills the attribution requirements of CC-BY-SA (some law experts
even doubt that), most authors uploaded there work here by themself
knowing how Wikipedia is going to use them. Therefore we have something
call an "implied-in-fact contract" that might legalise the use inside
 https://creativecommons.org/licenses/by-sa/3.0/legalcode (Section 4c)
And even under this strict reading of the license, the original post refers
to a blogger "who used a foto, with backlink to commons, and attributing in
mouseover", i.e. attributing _on the same webpage_ (together with linking
to the image source with full credit and license information), even though
not visibly without pointing the mouse on the photo.
Afaik there was no proper attribution on mouseover only a backlink to
Commons. And according to a recent judgment of a court in Munich it
is not even sufficent to provide attribution via mouse over anyway.
Wikimedia-l mailing list, guidelines at: