Honestly, even with a context-based argument, it's really tough to argue that any card art currently in game is outrageous with respect to what it's based on. The Witcher isn't necessarily "adult-only" in nature, but part of its draw is the fact that harm and violence are blunt and visceral - the world of the Witcher brings out the worst in some of its characters. I'm sure it would be easy to put together an entirely PG version of Gwent, but then it wouldn't quite be the world of the Witcher. This isn't even a world with gratuitous adult themes such as Game of Thrones, it's just that when you're designing the art of a character to represent them - as you say - without context, it's important to express who the character is in that one frame. I mean hell, you have Henselt burning Sabrina Glevissig alive in the background.
To be honest, I'm not sure you could have a "kind" depiction of Radovid at all and still let him be represented as himself.
As far as triggering is concerned, it's obviously subjective, and if it's having a negative impact on someone, it's moreso on them to handle things a certain way rather than censor the original work. I'm epileptic and have to close my eyes whenever games flash excessively on my screen. Were it up to me, I'd pass on all the unnecessary flash and strobe, but I make do.