Personally, I'd leave them - they are part of the history of that piece. I love news photos that are marked up! For grading, we (Beckett) do not count off for them because it was the intention of the item that they would be marked, but we specify on the label that were "editorially used".
If you still want to try, buy a lot of some junk photos - you can always find tons of random photos that are of little interest to people, for pennies. Try several things... I know I was able to use Goo-Gone on a comic book that someone used a grease pen on. Worked well, and made the comic smell orangy fresh, too!
