Commander was a class name in 13th Age, Captain was used in several Warlord Paragon Paths in 4e, Marshal was a class in the 3e Miniature's handbook.Those Google images are hardly a fair comparison. Warlord has already (and relatively recently) been used as a D&D class name where none of the others have, 1e level titles notwithstanding, thus it's far more likely you're going to find fantasy-based images for Warlord than any of the rest.
They just all have much more prevalent modern/RL meanings. Plus, Marshal is a name.
Happily. Google lets you exclude results, so, here you go, less pages that mention any of the common variations on "Dungeons & Dragons":Had the 4e class been called Marshal, say, or Captain, then it'd be that name that pulled the fantasy images while Warlord would pull images of various historical dictators and despots.
Ooh, one picture of a 'mech.