A nice official from Postcrossing that found its way to my mailbox yesterday!
The name of the picture/photo is Artists on the Thames (1947). It says: “The Wapping Group of Artists spend their Friday evenings in summer on Bankside, sketching the wharves, piers, bridges and ships; in the foreground is the group’s first president, Jack Merriott. The Wapping Group still exists, and remains devoted to outdoor painting, meeting on the river (from Henley to Medway) from April to September. It holds an annual show at the Mall Galleries.”
And look at this lovely hedgehog stamp!