As he relates in his book Michael Whelan's Works of Wonder (Del Rey, 1987), in the early 80s, Whelan attended a conference with Judy-Lynn Del Rey at the Del Rey / Ballantine offices to discuss cover artwork for the six volumes of H. P. Lovecraft novels and stories that were going to be issued in paperback. There, he learned that Del Rey was too cheap (my term, not Whelan's) to pay for cover art for all seven volumes. Instead, they had a budget for only two covers.
Whelan proposed making two large, panel-sized paintings, with each panel designed as a triptych. The individual book covers for the six mass-market paperbacks (published in 1981 - 1982) were derived from the triptychs.
The Del Rey trade paperback compilation, The Best of H. P. Lovecraft: Bloodcurdling Tales of Horror and the Macabre (1982), used the entire two panels for its wraparound cover.