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davidcyclebackIf it was just a photograph, say depicting WWII, the back notes by a well known WWII historian would add value and interest. If it was a wirephoto of the Chicago Bears, football notes from the GM would also likely add value.
I have no opinion as to whether the writing or add or detract from the card, but for most non-card original photos relevant back writing by a the photographer, editor or famous historian on a black backed card would usually be considered desirable. A todler's crayon doodles that would lower value.