[perezfan]

Depends upon the type of marker used. If it's the typical "grease pen" the marks could easily rub off and disappear simply by rubbing it with a dry soft cloth. Sometimes remnants of the marker will remain, or possibly an imprint/indentation of the marker. But it should still look infinitely better than before.

Give it a try before using any chemicals or wet substances that can compromise the photo's surface. Just "polish" the marked areas lightly with a cloth diaper or similar soft cloth. Do not use a paper towel or napkin, as paper could leave scratches on the surface.

Good luck, and let us see your "before and after" results. But be sure you really want to remove the marker in advance of doing anything. Sometimes the edit markings can tell a cool story and/or demonstrate how the photo was used for publication.