I was looking for the same thing. I wanted to adept nextgen gallery to an advertising slideshow banner with a link to the specific website for the ad.
I managed to adept the gallery with a single line of code:
in the file nggextractXML.php I added the next line:
echo " <info>".strip_tags(stripslashes($picture->description))."</info>\n";
Just before:
echo " </track>\n";
at the bottom of this file:
Now I can write an url in the description block of the image and when I click the image it opens the website inside a new window.
Make sure you mark the option Show next image on click on the Options -> Slideshow page of the nextgen-gallery
