It could be done by double exposure if he was able to rewind the film and expose it a second time. All he would have to do is ensure that only light in an alien pattern exposes the film initially. Then it is rewound and the second picture is taken. Presto, you have an "alien" in a picture. Very I used a similar method in high school to produce a picture of one of my friend's ferrets and a picture of a cow. I made it appear that a 15 foot ferret was getting ready to eat the cow:-p All that I had to do was manipulate the negatives. Like I said, I did this in one class period when I was goofing around in a high school photography class. If this person was determined, they could easily set this up.