I've noticed all the changes to the Stella character, too. The one that graces our FFXV theme background is a lot different than all the other images of her. This one makes her look like a kid, whereas the others make her look more mature.

I've tried looking up something about this & only found random thoughts. A lot of people think Luna & Stella are either the same or sisters (as some do here). Others say Luna is actually the dark-haired girl, while others say the dark-haired girl is "Himefu". FFWiki still has the dark-haired girl as "Unknown". The general consensus is that Luna is connected w/ Noctis, possibly as a love interest.

Of course, that's all nerd speculation.