Build background

function _M:buildBackground(group)
    local w, h = 200, 200
    local W, H = math.floor(display.viewableContentWidth / w), math.floor(display.viewableContentHeight / h)
    for y = 0, H + 1 do
        for x = 0, W + 1 do
            local img = display.newImageRect(group, 'images/background.png', w, h)
            img.x, img.y = x * w + _L, y * h + _T
        end
    end
end