On the Recycling of Cover Artwork
I mentioned in that post that I'd previously seen an image of a book by Andy Offutt that also sported this cover art. I hate not being able to cite things, so I headed over to Google Images to see if I could find it.
And I couldn't. But what I did discover—once I'd taken another run at the problem by cropping the lettering off the art and resubmitting the search—was that the image also appeared as cover art for a Canadian-based thrash metal band called Exciter, on a 1985 album called "Long Live The Loud."
(It also appears in a Pinterest collection of Real Dumb Album Covers, but that strikes me as unnecessarily judgmental.)
At any rate, examining that album's Discogs page a little more thoroughly, I found to my delight that they've got an artist credit there! And on finding it, I headed straight off to check my copy of the Methuen paperback for any possible confirmation... and sure enough, there it was (though in a place I somehow previously missed checking: the back of the book) So now I've finally been able to amend Fire's page at DianeDuane.com to add a credit.
The artist turns out to be Alan Craddock, who's gone on over the years since the above painting to do a whole lot of cover work on all kinds of British SF. He's now a much-respected colorist on 2000AD and elsewhere in the comics world—for both Marvel and DC, as well as various other comics, including some of @neil-gaiman's mid-90s projects for Tekno Comics). The above work would've been one of his very earliest ones, done while he was just starting to settle into his craft. (While in the meantime doing what a cover artist must do: whatever the art director tells him. So I willingly forgive him the metal booty shorts and spear-spiked heads and so forth.) :)
Anyway, there's a mystery unexpectedly solved! And happily, too.