Display.save image not post on faceBook Wall

local function networkListener( event )
if ( event.isError ) then
print( "Network error!")
else
print ( "RESPONSE: " .. event.response )
end
end

postData = "color=red&size=small"

joinGroup:insert(boardGroup)
joinGroup:insert(mainGroup);

local baseDir = system.DocumentsDirectory;
display.save( joinGroup, "screencapture.jpg", baseDir);

local ltn12 = require("ltn12")

local path = system.pathForFile( "screencapture.jpg", baseDir)
local imgfile = io.open( path, "rb" )
local imgData = ltn12.source.file(imgfile)