I wonder if these were made by three different illustrators because there seems to be a different signature on every poster.
In the AitD 1 Poster, on the lower left side, there seems to be the name Eagle, but I am not entierly sure. That name doesn't show up in the manual.
In the AitD 2 Poster, the name CALL can be seen below the steps on the right.
In the game's manual, you can find the name Greg Call under "Special Thanks".
And the AitD 3 Poster: I'm not sure if that is a signature or my brain is misinterpreting something, but below the right boot of Carnby, there seem to be the letters GB engraved on the floor. Sadly I can't find the full name in the manual.