HTML5 Tooltip via css and data- field works on link but not on image

HTML5 Tooltip via css and data- field works on link but not on image

following the tutorial http://ejohn.org/blog/html-5-data-attributes/ I
wanted to adapt the process of creating a tooltip for images (not links).
So I adjusted the "tooltip" class rules to be generic and not a tag
specific.
I reproduced my efforts in the js-fiddle http://jsfiddle.net/AqPN8/
As you can see in the fiddle it does work for links but not for images. Do
you have any idea why not?
Technically I think that :hover and alikes should also for for <img> and
not only <a>.