It's taken me nearly half an hour to research an answer but I can now tell you that the image you've linked to is from the 2004 Puffin edition. Puffin have been publishing the book since 1942 (only 2 years after Penguin books created their children's imprint) but the original publisher was Frederick Muller, with the book first delighting children in 1937.
As far as I can make out, this appears to be the original cover (although, since it's taken from a later reprint, I can't be absolutely sure):
http://www.biblio.com...12x/171874912-0-x.jpg
(Use your mouse to grab the image, to move it around, so that you can see it all - or simply zoom out).
Chris