A little more information.
After debug, I have figured out that the Doubleclick javascript that they ask us to put in the header is conflicting with the next gen gallery.
We take the Doubleclick javascript off the page – nextgen gallery works. We add the Doubleclick javascript into the page – nextgen gallery stops working.
Now that we know the problem – does this mean I have to choose between Doubleclick and Nextgen gallery, or can they be made to work together?