While in fact I was looking for the act on the license plates in the Royal Gazette, trying to find more information if and why the district Betong is the only district which has its own license plates, while otherwise it is only the provinces which are listed on them.
But what I found instead were announcements which specify a background graphic for each province, to be used instead of the plain white license plates. Yet so far I did not notice those colorful plates, this may be either because it's relatively new (the first provinces got a graphic in 2004, and still this year there are new ones) or just not much popular because of a higher price. Thus I have no photo of such a plate to show here, but will of course look out for one when I am in Thailand next time.
But nevertheless these graphics are interesting, as they display significant things of the province - landscape, culture or historic places. So somewhat similar to the emblems, which however are much more stylized and use less recognizable symbolic things. For example the of Surat Thani (Gazette) shows the Ang Thong archipelago, one of the natural highlights of the province. And funnily the view is exactly the one from the hilltop of Ko Wua Ta Lap, where I went once already.