File:Overlay Image (transparent black/white)
Use this feature to overlay a loaded/opened image over the image on the Canvas. Any part of the loaded image which is pure white or pure black as the case might be will allow the image on the canvas to show through.
Edit:Showthrough white only
This only applies if you have copied an image through the Edit menu. When you choose this option, any part of the image currently on the Canvas that is pure white will allow the copied image to show through.
So, you see, these two methods work in the opposite direction to each other.
