Pray for the spouse, but they need to talk about why the person is looking at pictures. Something may be missing. Many times, the spouse may want to be free with their loved one, but the other person has some reserves about things. If this isn't checked, after time, it can develop into desires for another person. There are scriptures that touch in these areas, and I'm sure someone will post them. For me, personally, as long as my wife doesn't want to bring another person into the room, I'm good with anything she can imagine.