The Horror in the Museum was the first Lovecraft collection I ever read. I wasn't this particular edition but it was a worn and yellowed mass market paperback. Now lost to the mists of time.
The Lovecraft covers really are some of my favorites in my collection. That is probably why I have more of them than any other author. This one in particular depicts the green stone statue of Bokrug that once stood in wonderous city of Sarnath. The good peoples of Sarnath had kept the statue as a trophy of their victory over the creatures that inhabited the nearby city of Ib. The lightning portends the return of the spirits of the slaughtered creatures to take their revenge on Sarnath and reclaim their icon.