Take a look of the beauty in the The Integral Trees, The Smoke Ring, Ringworld, The Legacy of Heorot, and Destiny's Road, all books by Niven, for example.
Robert Silverberg's stuff is really gorgeous - his Majipoor book covers are wonderful. Lord Valentine's Castle, Tales of Majipoor, Valentine Pontifex, the Mountains of Majipoor, River of Stars.
Orson Scott Card has some lovely covers.
Some of John Varley's covers are amazing.
For women sci-fi authors, Marrion Zimmer Bradley and Anne McAfferey have some really nice art, but less abstract, which is what I think you are going for.
Ignore the art from the 60s and 70s - that nude stuff was a long time ago.