Somewhere I have a newspaper clipping with a picture of what is appears to be a man riding a zebra. When I find it again I'll scan it in and put it here...
This is (I claim) a photograph from Peter Hill Beard's book The End of The Game (Hamlyn, 1965), showing Dr. Rosendo Ribeiro "making his rounds" on a zebra, which I have scanned in from a newspaper clipping.
But I might have made it myself in Photoshop ('All of your zebra belong to us...'). I wouldn't believe anything I read on the Internet. It's probably best to check the primary sources yourself.
See the Guardian's Notes and Queries page on Is it possible to ride zebras the way we ride horses?
Zebra-riding in art and literature
The cover of Chaosium's H.P.Lovecraft's Dreamlands supplement for Call of Cthulhu [subtitled Roleplaying Beyond the Wall of Sleep], depicts a zebra-riding gent with top-hat and telescope
There's a picture by airbrush cheesecake artist Olivia de Berardinis of a zebra-skinned woman riding a zebra.
[more as and when researched...]