I would lean more toward writing the id lightly in pencil over affixing any kind of label or post-it to the back. Graphite is stable over the long-term and consistent with what has been used historically. While the label or post-it is probably okay on most cases, the long-term effect is less-sure. If in pencil, I have a hard time believing anyone could distinguish between modern pencil and vintage (unless they happen to recognize your handwriting).
You might also consider just putting them in sleeves or toploaders and putting the label on that rather than the photo itself.
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