Sharon,
Have you tried spinning the picture through 90 degrees and scanning it again? I have just been working on a picture of my grandfather which originally looked like this when I first scanned it, but when rescanned at a different angle, almost all of the smaller marks disappeared and left a much clearer picture. I wouldn't guarantee that anything will change on yours, but it certainly worked for mine. And it beats sitting there clicking away endlessly trying to clone them away.
Glen