My original tumblr post was here. These covers by Ed Valigursky were posted by Modernizor and attributed to a blog The Geeky Nerfherder. I don’t know for certain exactly which post was the source, although this one, which has a huge number of images, is a likely candidate.
I hate to admit it, but I remember reading some of those issues when I was very young. My parents subscribed to severalSF ‘zines, and I started out with a short story “Callahan and the Wheelies.” Beat the hell out of Whinnie the Pooh!