In the letter she explained that she had slept with two guys while he had been gone, and she wanted to break up with him and she wanted the pictures of herself back.

You gotta love a man like this. (Humor in the face of defeat.)

 So the Marine did what any squared-away Marine would do. He went around to his buddies and collected all the unwanted photos of women he could find. He then mailed about 25 pictures of women (with clothes and without) to his girlfriend with the following note:

" I don't remember which one you are. Please remove your picture and send the rest back."