There are those who think that an outfit with a low neckline is next to pornography.  There are also those who find nothing at all objectionable about The Birth of Venus (the artwork).

Being very well acquainted with the male mind, let me clarify.

There are plenty of ways of laying down that don't stir up imaginings.  At the same time, there are photos of girls who are standing up that can lead to these inappropriate thoughts.

This would be a case where the photographer and the subject would impact whether or not the photo is immodest, like all photos.  If the image comes across as inviting for immodest purposes, then it becomes an immodest photo.  If, on the other hand, the subject is simply smiling and giving a sense of humor, then it becomes modest (it's a given that the clothing is modest already in these examples).

Also, and here's where the line gets fuzzy, the intention of the viewer of the photograph has something to contribute to it.  One may look at a photo of a modestly dressed girl lying down and think it's a marvelous tribute to the beauty of women everywhere.  Another may look at the same photo and begin to undress her in his mind.

I hope that is a little clearer than I think I wrote it.