It’s about ‘alternate timelines’ and ‘what-if’ questions, I guess.
I’m not very much into BG, but many of those portraits I’ve seen are more or less variantions on some kind of theme… As for the latest ‘sparkle guardian splendour’ — I’m not impressed by those anyways.
Mercenaries, however, is another matter — although admittedly skins are just art, and sometimes they get reused in another game mode, including this one, where I’ve found out that some portraits come from the corresponding BG arts — and there’s one I don’t quite get: What is Baroness Elise (in Mercenaries)?
Wait a bit, are we talking about (elven) women or girls — who’s the fairest of them all — or class-specific portraits? 
I’m a bit confused.
As for Liadrin, the portrait you’ve chosen for for forum, if memory serves, is obtained from WoW, which I don’t play, so it’s out of my reach anyway. However, with KotFT cards added to Core during the ongoing event, I’ve finally done the Lich King Missions (I’d quit the game when they were current), using some of these cards for the final challenge, and unlocked that Arthas paladin skin — to me, it looks more impressive (apart from that, speaking of alternate paladin portraits, I previously bought Second War Uther — again, with gold, which is how I do it with purchasable heroes… Sometimes purely for the soundtrack from WC3, if nothing else; ironically, that musical piece was added to the Alliance in TFT, when Uther was no longer around, but still…), and Liadrin isn’t my type anyway.
As for priests, well, the original Tyrande was (or maybe still is) given for free to everyone — that’s something already.