get("#356") -- num2hex
get("#348") -- rgb functions

function hashImage(img)
  local hash = 0
  local w, h = img.width, img.height
  
  for y = 0, h-1 do
    for x = 0, w-1 do
      local pix = img.getInt(x, y)
      local r, g, b = rgb(pix)
      local p = r+g*33+b*71
      local scale = bit32.band(y*w+x, 0xFFFF)+1
      --hash = (hash * a + pix) % 0x100000000
      --a = (a * b) % 0x100000000
      local arg = (p+1)*scale
      hash = bit32.bxor(hash, arg)
      --[[if x < 10 then
        print("scale", scale, "pix", pix, "p", p, "arg", arg, "hash", hash)
      end]]
    end
    --print("y", y, "hash", hash)
  end
  
  return num2hex(hash, 8)
end

Probably fairly crappy (might cause hash collisions), but I guess it can be used for now