For U.S. Passports, there is a nice guide to how your picture should look at:
They suggest that the centerline for the eyes should be 1-1/8" to 1-3/8". They suggest that the face should be about 1" to 1-3/8" in size. And that the total picture should be 2" square.
After playing with PassportPicture for about an hour, it looks like it will do a good job with the last requirement. And it will help with the second. But it provides nothing for the first.
It would be nice if the developer could add a couple of guidelines to aid in meeting the first requirement. And it would be nice if you could have at least two of both the existing square and the new guidelines. That way, one could express a range. I.e. the face should fill the inner square, but not exceed the outer.
This program isn't bad. But with just a few minor enhancements, it could be great!