If I were to guess, I'd say that there were different photographers, and the one who was supposed to take Sanji's image made the mistake. Some bounty posters were taken during the raid on Enies Lobby (notice the posters of Sogeking, Robin and Franky).
But Sanji was not there to begin with (busy closing the Gates of Justice), Nami was shocking everyone with her Perfect Clima Tact who got closed to her and Chopper was temporarily paralyzed on the ship. Hence only Sanji, Chopper and Nami were left for the bounty photos, Nami's photo was then taken by a magazine reporter (who may or may not be at the party, but can't go to Sanji or Chopper with that excuse), Chopper's photo was definitely taken at the party (notice his expression), however Sanji was busy with keeping up with the food demands during the massive party (can't have a cook's image on the bounty poster).
As for how nothing is mentioned. I would say he was nervous and made an honest mistake.