Here's a photo that was just posted on the Scottish Register of Tartans facebook page. It was apparently painted in the year 1792 by John Brown, and is an image of King Fergus I, who according to the script over the image began his reign in the year 3641 anno mundi . I haven't taken the time to try to convert that to the modern calendar, but it is several hundred years "BC".

Take a look at the kilt. From what I can see it is not even a great kilt, but a tailored kilt, accessorized with a cloak of the same tartan material fastened at the neck by a brooch. Feel free to correct me if I am mistaken, but I don't remember seeing historical images of great kilts fastened this way, which is why I'm thinking that the cloak is intended to be a separate item.

Interesting since the earliest known tailored kilts date to the very late 1700's.

Also note the bonnet, which is certainly something that old King Fergus would never have worn. And is that a basket-hilt sword?

So this appears to be an artist in the 1790's placing contemporary clothing and accoutrements on a very, very old Scot. Really interesting.

Looking forward to folks more knowledgeable than I chiming in on this...