First, when something is reported in a window there is often a light source. In some cases, it is a streetlight. But there is a large light, the moon, that is always there.
Next, windows with screens act like multiple diffraction sources that interact with each other. And a bent screen, and few are taut, can highlight some images.
Another possibility is dust on the window that has points are streaks that diffract light.
Finally, consider the reflections, including the internal reflections, of light making things fuzzy. And with older windows the thickness is not uniform, making the reflections and refractions even more complex.
Internal reflections can highlight a single color, due to a certain wavelength reinforcing itself. This can be seen with floating oil, which shimmers in colors due to the thickness of the light path through the oil.
So, Are Those Images of Jesus? Do we see ghosts? One can argue either way.
If it is what God uses to have a person believe the image is of Divine origin, we cannot question it. God can communicate in a multitude of ways, and this could be one of them. Then again, we would have no way of knowing.
So, physics provides a possibility, but the spirit world offers others. Are the appearances what we see, or are they what light makes us believe? This I cannot answer.
The physics is only a cursory approach, a true understanding is too complex for this venue. But it is possible to present the qualitative ideas, which is the purpose here.
The introduction image is our own Zazzle product, The digital image is my own.