Dang this is a hard one as there are so many to choose from!
Easy answer would be anything by Larry Elmore but that'd probably be cheating although still accurate. Especially Elmore's early work for D&D and his Dragonlance work.
Overall though there's been that much different art from RPGs themselves and magazines such as Dragon and White Dwarf that have either triggered a purchase response or scenario idea response.
The one I'm going to go with though did none of that, the cover to White Dwarf issue #61 by Chris Achilleos. That's the original #61 not this renumbering malarkey they've done since then.
Why? Well nothing says RPGs to more than a whateverthatis wearing armour and wielding a whip.