Just playing around with cover ideas here for The Witches' Stones, relaunch of Book 1.
Science: Nothing says science quite like a picture of a planet and moon orbiting a neutron star (pulsar).
Science Fiction: Add a space ship transitioning from normal space to Omega space, now you have Science Fiction.
Science Fiction Romance: Add a dark haired, slightly witchy looking beauty and now you have SF Romance. And make the pulsar flare dangerously, to add a note of danger.