Start with meaning, then look at clarity. Choose the photograph you would miss if it disappeared, not simply the one with the most polished pose. For a small pendant or charm, a face should still be recognizable when the image is reduced. Photos with some space around the subject give you more freedom to reposition the crop.
Natural light is helpful, but an older photograph can still work. We can discuss gentle cleanup, background removal, or restoration before production. Those changes should always be shown to you rather than silently applied.
Finally, choose the shape that supports the photograph. A round pendant feels especially natural for a close portrait, while a square pendant can preserve more of a family scene. The right setting supports the photograph instead of competing with it.