A mirror literally shows you a “mirror image” (left is right) while a photograph doesn’t. Most people see themselves in a mirror a lot more than they do in photographs (selfies don’t count, most front facing cameras show a mirror image) so they get used to seeing themselves mirrored. So, when they do see an actual photograph, it just doesn’t look “right” because it’s not the mirror image they have become used to seeing on a daily basis
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