Being as I don’t work much with the ”png” format and I thought the original post was asking about the overlay mode I could not give an honest work around to the “png” overlay problem you’re experiencing. If I understand correctly, I would like to point out however that if you do as Thundering suggests with cutting out part of the image, that as long as the image size remains the same as the original and the cut out is not moved then you merely have to hold down the “shift” key  while moving the “png” file onto the “tiff” with the “move” tool it would center automatically as a layer and thus no need to use the nudge keys (arrows).