I just started using NG Gallery and was having the same problem - and found this thread. When opening the gallery my menu drop down bar stayed OVER the slideshow:
I upped the z-index on all the Nextgen CSS elements, but it didn't work. I made my z-index on the drop down -1, which made it completely dissappear. After more fiddling I finally made the drop down menus z-index -.5 which finally worked.
I'm not a CSS expert, but it worked. If someone wants to explain to me why I'll be a wiser man.
I have to check in other browsers at home, but hope that might help - james...