Pay close attention to the file extension of your pictures. I have found several girls with pictures (like the profile pictures for Tsunade, for example) that won't load because they have the extension ".jpeg" instead of ".jpg". If a file has the ".jpeg" extension, the game will not load it or any subsequent pictures of the same type (For example, if "Sex1" has .jpg but "Sex2" has .jpeg, the game will load "Sex1" but not "Sex2" or any of the "Sex" pictures after 2).
If you want to use a picture with the ".jpeg" extension, simply rename it ".jpg" and it will work.