1e: Gygaxian, gritty art.
2e: TSR fires Gygax. The company wants a more High Fantasy feeling, and wants to tell stories with adventures, settings and related art. Elmore, Caldwell, Easley and Parkinson are paid to produce such high quality work. Larger-then-life, over-the-top art, it was good to illustrate legends and epics and fables. Later in the edition, the settings were bound to the art of one or two artists: Mystara had Elmore and Caldwell, Dragonlance had Elmore, Dark Sun had Brom, Planescape had DiTerlizzi... Moreover, for the first time, the art preceded and inspired the world-building (see the case of Dark Sun: Brom draw, and then Timothy Brown translated the art into setting lore...)
3e: The change of edition implied a drastic change in art also. Covers looked as tomes now. The overall atmosphere was provided by Lockwood. From the stuff of myth to a more "realistic" fantasy, precise, efficient and codified (Lockwood Dragons).
4e: The art was video-gamey, colorful, full of action and dynamism.
5e: nostalgia for 2e and its fairytale atmosphere. Mike Mearls finally understands the importante of art (both in quality and quantity) to sell physical books.