Hello, I have the same issue Horst had, but adding the title tag to the link in the code within the page did not work for me.
Please see the example here: http://icanexplainit.com/not-defined-issue/#!prettyPhoto
after inserting the title tag in the code (I tried several different locations within the code) the photo will open as
http://icanexplainit.com/wp-content/uploads/2013/02/post-9.jpg instead of within the “prettyPhoto” window.
Thanks in advance for your help.