Thanks Christy for raising this objection. It is quite relevant for this thread and I answer it here with pleasure (also because the original thread where you posted it is locked):
According to my explanation all cultures today that share a human body are Image Bearers. Therefore, even if they do not have writing systems, they are NOT less image bearers or less moral.
The reason for this is that at a certain time T in history God made the first humans in His Image (Genesis 1:27). By this act God definitely chose the kind of body His Son would take at Incarnation: Jesus Christ “is the image of the invisible God, the firstborn over all creation” (Colossians 1:15). In that way the human body became the sign of being Image Bearer.
It is obvious that before time T no population can be said to be in God’s Image.
So the crucial question is:
How do we ascertain this time T when God makes the human body in His Image?
It seems to me we all agree that at the latest time T can be set when Writing appears.
If someone can provide convincing reasons to predate the moment when God makes the first Image Bearers I will wholeheartedly change my view. Nonetheless the discussion in the original thread of your post shows pretty well that Language cannot be considered a clear sign for the creation of the first Image Bearers, no matter whether they were a single couple (as @agauger assumes) or a population (as I assume).
So for the time being the safest solution seems to be Writing.
Once again:
Today all cultures, also they without Writing, are Image Bearers.
Before God created the first population of Image Bearers (at the latest at the time when Writing appears) no creature was bestowed with capability for freely loving God and therefore no creature was accountable toward Him and capable of sinning.