On cover images filling in gaps... beware of this approach. I've seen some wildly inaccurate cover art out there (especially on reprints - some of the Pernese dragon renderings, I'd swear the artist had never read the books, as they don't even come in those colors. And Stephen Brust's Jhereg books - the MC makes a point of growing a mustache because the inhuman species he works among cannot grow facial hair, so it's an integral part of his rebellious character, yet I have not seen a single cover depict him that way.) You can't always luck out with an artist like (now-semi-retired) Micheal Whelan, who actually takes the time (and is allowed to take the time, 'cause he's Michael freakin' Whelan) to read the manuscript in its entirety before designing the art. You might get an artist who gets a brief - or, worse for you but better for the bottom line, stock art. And, yes, it does color how your reader "sees" your characters and aliens/creatures and such, even if your descriptions counter the cover.
Tangentially, beware of relying too heavily on human analogues. A late author acquaintance of my parents (F. M. Busby) had aliens (the Demu) he described as being vaguely "lobster-like," noting specifically that they did not look like lobsters nor were they colored like lobsters, but there was some aspect to them that made humans think of them that way. That description, unfortunately for him, opened the floodgates: lobster-like cover art, lobster gifts at cons, lobster this, lobster that. He hated it... but, from a reader standpoint, clearly it made an impression: these aliens are like lobsters. And I'm doubtful it interfered significantly with the overall story. It just wasn't the image he wanted to instill. (I never did think to ask how he would've described them differently, had he known the fallout of his original description.)
As for describing aliens without human terms, Orson Scott Card touches on this on his writing book (Writing Fantasy & Science Fiction); you might take a look at his advice. As others have mentioned, beware not to overdo it, and try to think in terms of how your characters would view each other, even with an omni narrator. No human is going to describe another human as an upright-walking bipedal mammal, mostly bereft of fur save longer locks on its rounded scalp and tufts protecting genitals, descended from tree-dwelling primates and bearing five long, blunt-nailed digits on each forelimb capable of intricate manipulation and numerous rude gestures. They're going to describe Bob as having dark hair and being tall enough to bump his head on doorframes, or note Janine's squinting green eyes, or compare Jake's round, flushed face to an exceptionally expressive tomato (for a few quick bad examples.)