z-index.lua

function zIndexCharacters()
   local function redraw()
      -- images are all held in an allImages array  
      for image = 1, #allImages do
               allImages[image]:toBack()
      end
   end

   local function compare( a, b )
      return a.y > b.y
   end

   table.sort(allImages, compare )
   redraw()
end

Runtime:addEventListener( "enterFrame", zIndexCharacters )