You probably got this from his discharge, in which case it's the unit he was assigned to at the time of his separation from the service. Period. Anything else would have to be determined through records.
The letter you refer to may be a copy of Eisenhower's pre-invasion speech, of which printed copies were widely distributed among the invasion forces. Reprints as well as originals have been available from a number of sources for years.
The picture, unless it's dated, captioned or otherwise reveals some solid info relative to the above (or any other) unit, could have been taken anywhere; in the U. S. (or England) during training, in Europe or even just prior to his discharge.
If he took part in the Normandy Invasion, it will be duty noted on his discharge.