Does anyone know the source of the custom of reflecting on the Rebbe's face during the krias shema al hamita Someone told me about this "minhag" is there a source for this one. what then should be the thoughts during reflection?
There's a concept of remembering the Rebbe and connecting to him by picturing his face or looking at a picture of him. The Rebbe spoke about it with respect to his father-in-law and predecessor, the Rebbe Rayatz, and chassidim extrapolated to the Rebbe himself. I'm not aware of a custom to do it specifically during the bedtime Krias Shema, but it's reported that the Rebbe encouraged it as a daily practice, and he implies in a letter that it can help alleviate nighttime fears.