I agree with you in part. I think it isn't only impurities, probably its a propertie of the arrange of the electrons of the glass. If its only impurities or electrons arrangement, the image is form from the fluorescent absorption, right?
I need help to understand what is happening when I project a image in a sheet of glass.
Imagine this picture, a projector as the light source projecting a image in a sheet of glass:
1: I have high transmission. (the light that pass away the sheet of glass)
2: Low absortion
3: A so so...