A couple of art inaccuracies that I remember are Sulu in a blue uniform shirt on the cover of Shadow Lord, and a large sword being prominently featured on the cover of Captain's Honor despite no sword being featured in the novel at all.
There are also plenty of New Frontier
novels with swords on the cover, even though there aren't any in the books. It's not inaccuracy, it's symbolism. Not all art is literal. (Although in the NF case the sword represents the starship Excalibur
. On The Captain's Honor
, it presumably symbolizes the ancient martial traditions that drive the Magna Romans.)
I always took the sword on the New Frontier
covers to be Calhoun's sword, the one that scarred his face and that he took from the man who scarred him.