You are absolutely right Anita, the cover picture was taken from
The Mountain of Adventure - a major blunder by Royal Mail who then stuck the Famous Five underneath it. The stamps were designed by Christian Birmingham, who is quite a well-known Children's book illustrator. You can see the images better on the postcards that the Royal Mail also produced, and there is a very nice presentation pack with a little booklet about Enid and her work.
The Faraway Tree picture isn't very clear, even on the postcard, it has a sort of dream-like quality to it:-
Despite the Tresilian illustration, I also thought a signed Society Famous Five cover would be nice, with all five signatures on it!
My apologies for the wide scrolling on this thread, but I thought you would like to see a decent sized image.