Security issue. You can provide wrong size of image. It will countinue read out of buffer range.

[vird]

some coffeescript to test
proper usage

#!/usr/bin/iced
png_encoder = require 'png'
fs  = require 'fs'

local_buffer = new Buffer 2*2*4
local_buffer.fill 0
size_x = 2
size_y = 2

img = new png_encoder.Png(local_buffer, size_x, size_y, "rgba")
img.encode (dst_buf)->
  fs.writeFile "res.png", dst_buf, (err)->

bad usage

#!/usr/bin/iced
png_encoder = require 'png'
fs  = require 'fs'

local_buffer = new Buffer 2*2*4
local_buffer.fill 0
size_x = 2
size_y = 1024

img = new png_encoder.Png(local_buffer, size_x, size_y, "rgba")
img.encode (dst_buf)->
  fs.writeFile "res2.png", dst_buf, (err)->

btw. other modules (node-jpeg, node-gif) also need checking.
png version 3.0.3