It turns out these were illustrated by a man named Alex Nuckols. It's beautiful, classically rendered, and downright classy work that gives my beloved Greg Irons-illustrated AD&D Coloring Album a run for its money. I know many gripe about the merchandising of D&D in the early 80s, but in my opinion, some of the best (if not the best) D&D-themed artwork of the era wasn't being produced by TSR at all. Nuckols' work for St. Regis is a perfect example.
There's a gallery of several Nuckols-illustrated St. Regis products at the Tome of Treasures forum. A lot of the scans and photos are frustratingly blurry, but even this limited glimpse is well worth your time if you're into D&D fantasy art in the least.
(By the way, I'd be more than willing to bet that some of these images - including the one posted above - were originally intended to depict scenes from J.R.R. Tolkien's work. That looks like Bard of Laketown slaying Smaug to me, and I'd be shocked if the "Death Rider" art wasn't supposed to be one of the Ring-Wraiths seeking out Frodo and company.)