Yes, it was a Christmas card design drawn by Hergé in 1977/78. It's reproduced in full in
Le Monde d'Hergé, but if anyone hasn't got it to hand, I've scanned it :
Hergé's Egyptian fresco, 1977 (187kb)
The Christmas card designs he came up with over the years are fascinating, they're imaginative, and some are really beautiful. The medieval illuminated manuscript, for example, or the silouhettes of Tintin & co. walking to Midnight Mass. It's worth checking out
Tout Hergé for a full series of the cards, albeit in thumbnail images.
Edit : Sorry Ed, double-posting again !