I can think of few other artists that have made such an impact on me as Iain McCaig. I first came across his work in City of Thieves, the Fighting Fantasy book, which was utterly spellbinding. As well as doing fantasy art, McCaig is a renowned concept artist, having worked recently on Guardians of the Galaxy and The Avengers, as well as the upcoming Star Wars Episode VII. He is well-known for having created Darth Maul and has also done video game work on Titanfall.
McCaig has a blog showcasing his art, although it hasn't been updated in 4 years. Still, his incredible art is worth using in any of your fantasy games as it's just so evocative and phenomenal.