Warhammer wouldn't be so epic without arts, which paint the battles of dark future in all their bloody perfection. Let's compare how the arts were changing during all the wargame's lifetime.
While early arts were bright and colourful, newer editions brought some grim darkness, made the colours much darker and serious and the battles more fearsome.
Usage of computer graphics seriosly changed the art's design. Nevertheless, some people think that they've lost a soul when they've stopped being drawn with a hand.
Some arts present xenos' in a really bright way, however, the most of them show brave space marines.
It's great how modern Horus Heresy includes the design of the old arts, making it more realistic. This lets to gently include old arts to this universe and explain, why they differ so much from the modern ones.
Here's the newest arts - the rumor has it that there are a lot of new models, coming in 2017 on it.