The only other z-indexes I use in this app are 1, 2 and 3.  With a z-index this high, if the user plugged in the usb cable to the computer, the back button would be visible on top of the overlay.  I think a zindex of 1 is high enough here.  Or, better, just move the button down in the html file so it is next to the toolbar. That should put it on top of the picture, I think.