As the person who drew all of those halos, there are two simple answers:
1. They look cool, I like drawing them, so I put them on people.
2. In-universe, I'd say it's a visual representation that the Pelorians use in their religious artwork, so it is included in this as well. The Halos on the Solar pantheon are a sunburst, and only important figures in that pantheon have them, usually some the highest ranking one or the most important in a grouping. Yelm is basically all halo on there. The Lunars probably use that instead to signify a direct connection to the Red Goddess - notice how none of the Seven Mothers (save for She Who Waits, of course), has a halo, but everyone who comes from the Goddess does.
Your concepts for a representation for the Pelorians, Yelmalions, and Lunars all make sense, but how would you personally think the Orlanthi would represent him? I have trouble with the idea that the Orlanthi of Dragon Pass, or those of the East Wilds in Ralios, would adapt any Pelorian or Yelmalion, or especially Lunar, imagery into their own representations.
Orlanth's look (and other significances of visual representation)
in Glorantha
Posted
Your concepts for a representation for the Pelorians, Yelmalions, and Lunars all make sense, but how would you personally think the Orlanthi would represent him? I have trouble with the idea that the Orlanthi of Dragon Pass, or those of the East Wilds in Ralios, would adapt any Pelorian or Yelmalion, or especially Lunar, imagery into their own representations.