The Call of Duty series is based on modern warfare, not WW2 era. And the last game that had an image of Hitler in it is Return to Castle Wolfenstein, which was based in WW2.
EDIT: I don't have an answer for you, but the answer given above is absurd. Of all of the Call of Duty games released so far (June 2009), only ONE of them has been set in the modern era. ALL of the other games have been set in WW2. People, please don't post answers if you don't have the slightest clue what you're talking about.
ANOTHER EDIT: The only time Hitler was presumably ever seen in any CoD game is in Call of Duty: World at War, where a video of him giving a speech to the German citizens is seen among other videos during the briefing of the 5th mission of the campaign. (which, technically, is the second mission for the Soviets)
Edit: There really is no realistic reason for Hitler to make an appearance in any CoD title (in terms of being in a level) because there was no real point at which a player in the typical Allied campaign would come into contact with him. Even at the time Berlin was about to fall/did fall, in 1945, which would be the closest any player would get to him geographically, Hitler was in a secret bunker. He wasn't known to have been there (by the Allies) until the discovery of his body after the war.
ANOTHER EDIT: There is a death card in a level, and it is by Hitler's dead body.