Looking through the Locus directory I was also reminded that I really like Melanie Delon's cover of Libyrinth by Pearl North, as well as her covers of Alison Sinclair's Darkborn and Kirsten Imani Kasai' Ice Song. The covers that feature a single character and not much else are becoming more common these days and I think it might be easy to discount that work for lack of complexity, but to do so would be to overlook the question of how effective and provocative the artwork is. With so much focus on a single figure and face it is especially important to make those images strong and I think Delon does a great job with her portraits. I am unsurprised to find that her work has been featured for years in such collections as Spectrum (she had the Cover of Spectrum 16), Exotique, Exposé and Fantasy Art Now and she was selected to be one of the authors in Ballistic's masterwork series on how to do digital art.
Finally, I particularly admire the breadth of Stephan Martiniere's work this year. He has done a few of his now-familiar visionscapes (always framed with an interesting foreground piece), but he also did the quiet personal cover of Jo Walton's Lifelode for NESFA Press, and the stunning cover of Ian McDonald's Desolation Road for Pyr's reprint.
So far these are the artists I'm sure I'll be nominating for Best Professional Artist for this year's Hugo Award, and I commend them to your attention.