The indicium on direct marketing mail has typically been mostly black text, rather boring. Ian Billings reports that there will be a trial of colorful stamp images to see if the effectiveness of these mailings can be improved. Ian highlights the image of an Alice in Wonderland stamp, but he also mentions a second-class Machin.
It is generally accepted that more recipients will open an item of mail with a stamp on it rather than a postage meter or text-based indicium. I believe this has been shown in comparative tests. Royal Mail will see if that extends to a stamp image printed on the envelope.