One of the things that struck me from the linked blog was ableness. I hadn't thought about it, but I'd love to see a few pictures of adventurers (and/or bad guys) who are missing a limb here and there - peg legs, a hook hand, an eyepatch, a literal "one-armed bandit", maybe even a disabled mage in a fantasy wheelchair or moving about on Tenser's Floating Disk. If not overdone, I think they would really help to expand people's imaginations into heroes they haven't contemplated before.
Also, in the ways of art I'm a fan of many of the old artists - Roslof, Ken Frank, Elmore, Easley, Parkinson. Most of it was because their art, while fantastic, seemed very much grounded in a reality you could believe in. I liked Jim Halloway's comical art, as well as the cartoons in the old 1E DMG.
Above all, as a personal favor, I'd love to see a 5th edition rendition of the "
fighter trapped in a water-filling room with skeleton" picture [that montage is done by [MENTION=9470]weem[/MENTION], BTW] - and perhaps homages to the old greats - "A Paladin In Hell", "Emirkol the Chaotic", Elmore's "The Dragonslayers", etc.