How do you clean old photos

Get a box that is larger than the picture and sprinkle a generous amount of baking soda over the bottom of the box. Put a baking rack over the baking soda. If you don’t have a cookie rack, you can just set a couple glasses or bowls in the box and then lay the picture across the top of the rack or glasses so that it is raised up off the baking soda. Close the box and leave it for 24 hours. If possible, put the box somwehere dry, but do not bring it in the house as you don’t want to spread the mold/mildew spores inside. The baking soda will dry out the mildew so that it can easily be removed.
After 24 hours, open the box and gently brush off the mildew using a soft paintbrush if possible or if you don’t have a paintbrush, a soft cloth can work too. The mold should easily brush off. If it doesn’t, put it back in the box. You are waiting for the mold to turn into a powder. Add more baking soda to the box if needed, or use a DampRid container instead of baking soda. If it is raining or snowing outdoors or otherwise very humid, you may need to bring the box into the house to put it somewhere the mold can adequately dry out. Just be sure to keep the box closed while in the house so the spores are contained.
If there is any staining that remains from the mold, cleaning the picture using the methods in the article. Good luck!
Source: Image Master Photo Repair – Destroy & Prevent Photo Mold & Mildew
Source: The New Orleans Conservation Guild, Inc. – Is your artwork wet or damaged? Here’s what to do.