This is just my personal answer, not based on Qur'an or hadis directly:
The idea of the pray is to concentrate on you yourself and Allah. So, anything disturbs your concentration is not good. Also, picture thing as you said is because to assure that you pray only to Allah and nothing else. In this respect, I would say, praying in front of the mirror would be weird as it may both hamper the concentration and seems as if you pray to yourself.
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