My original tumblr post was here. Image is a paperback cover that contains the text “Triax. Edited by Robert Silverberg.” This image was posted first by Feeling is First who added the text “Fontana paperback, 1979. Cover artwork by Justin Todd.”
Tumblr favorite #1992: Triax
I can’t find a straight image, but here’s a link to a page selling a copy of the American edition (1977), along with a little more information about it:
http://www.abebooks.com/servlet/BookDetailsPL?bi=18093208908&searchurl=isbn%3D0523401213
As always, thanks for the link.