I assume that you are discussing the cover of the book "Cinquante Ans de Travaux Fort Gais" which depicts Herge hard at work at his drawing board, wearing a prison outfit and a ball and chain! His characters surround him, making sure that he is getting on with his work - Tintin is even brandishing a cat o' nine tails!
Here is an alternative explanation:
That cover was used for issue 174 of the weekly comic "Journal de Tintin", published in the second week of January 1979 in order to mark the 50th anniversary of Tintin's first appearance. This issue also included a life-size poster of Tintin and Snowy.
This poster can be seen on this auction website
(but no longer for sale).
The actual comic and the poster are for sale on ebay
This cover was used again for a limited edition book in which Herge describes drawing the page of a Tintin story, from planning to rough sketches of hands, heads and other details to the actual drawing and colouring of the final page (in this case a scene from "Picaros" featuring Sponsz and Alvarez which was not used in the final book).
According to Bedetheque
(the IMDB of French-language comics), 2000 copies were distributed during cocktail celebrations of Tintin's fiftieth anniversary.