Michael Whelan's Barsoom covers

I mean, he's no Frank Frazetta.  On the other hand, he did a better job than Frazetta did of actually following the descriptions in the books.  He just didn't make them nearly as exciting.  But Michael Whelan is a highly regarded and prolific book cover artist in his own right, who's done a ton of covers that are highly recognizable and should be.

In order: A Princess of Mars (1912)

The Gods of Mars (1913)

The Warlord of Mars (1914)

Thuvia, Maid of Mars (1916)

The Chessmen of Mars (1922)

Master Mind of Mars (1927)

A Fighting Man of Mars (1930)

Swords of Mars (1935)

Synthetic Men of Mars (1939)

Llana of Gathol (1941)

John Carter of Mars (combines novellas John Carter and the Giant of Mars (1941), attributed to ERB's son, and Skeleton Men of Jupiter (1943)