With the Blish novels, few of the painted covers had much to do with the episode content, save for James Bama's beautiful NBC promotional art for WNMHGB used for novel #1, or ST 7--.
Except that WNMHGB wasn't adapted in the first book, it didn't get adapted until #8.
As I said, the Bama poster was commissioned by NBC as promotional art for the premiere and first season of Star Trek in general. It wasn't created specifically for "Where No Man," but was meant to represent the series as a whole. It just happened to be based on imagery from WNM because that's what was available to James Bama at the time the artwork was commissioned.