If it is a static page there is a pretty easy fix.
Open up /nextgen-gallery/widgets/widgets.php.
Edit the line (currently 322) that says
//TODO:For mixed portrait/landscape it's better to use only the height setting, if widht is 0 or vice versa
$out = '<a href="' . $image->imageURL . '" title="' . $description . '" ' . $thumbcode .'>';
And replace the $image->imageURL with your URL (in quotes).
So it becomes
'<a href="' . 'YOUR URL HERE'. '" title="' . $description . '" ' . $thumbcode .'>';