These illustrations were originally created for Achebe's US covers, from Anchor, where there are a couple of extras to be found. The designer credited is Edel Rodriguez.
I really like these images. Arrow of God and No Longer at Ease, in particular, remind me of the work of Saul Bass (see this post for more)...
Penguin's catalogue for 2010 appeared online briefly over the weekend, before reverting to 2009. While it was up I managed to grab a few other covers, which I'll post in upcoming days.
UPDATE: The Achebe cover designer, Edel Rodriguez, got in touch to say "I did all of the illustrations and hand lettering for the original versions. I had to rework the images to fit them in the Penguin UK templates. They have a certain type treatment and color palette that is part of their format. Naturally, I prefer my originals, more space, less crowded, etc. But I understood why things had to change for the Penguin UK versions."