Some were painted by Frank R. Paul: there's a gallery of these on Frank Wu's FRANK R. PAUL's ARTWORK CHECKLIST PART I: FRONT COVER ART page. In the book 100 Years of Science Fiction Illustration, Anthony Frewin writes that:

Paul had little or no precedent from which to gain inspiration and it is a fitting tribute to his incredible imagination that his vision and stylisation of SF would characterise all similar work for the next 40 years. Paul, when illustrating a story, created those monstrous galactic cities, alien landscapes, and mechanical Behemoths entirely himself — the descriptions contained in the stories were never ever much more specific than, for example, something like, 'shimmering towers rising into the clouds from a crystal-like terrain'.

Frewin notes that Paul's aliens tend not to be frightening: "their appearance is friendly and domesticated almost". That's certainly true of the dinosaurs menacing the submarine on the February 1927 cover. Their goofy grins remind me of Sid's Snake from Whizzer and Chips. The betentacled secretaries that Paul drew for an episode of Baron Munchausen's New Scientific Adventures in a 1928 Amazing Stories frighten me more.

But, to quote Frewin again:

To say that all of what follows in the present book is a testimony to Paul's unique creativity would not be hyperbole. No other individual enlarged so greatly the vocabulary of twentieth century popular art in its attempts at grasping the future...