Which one is correct: "pressed his fingers against the window" or "pressed his fingers to the window"? Or is there no difference in meaning?
You would press "against" and point "to" the window. You would not press to, nor point against the window.
That's the difference. However... in thinking about this you could actually get away with saying "He pressed his fingers to the window." The reason being, in some sense, that "to" can have the connotation of "gently against", but it must be used carefully!
Consider "He pressed his ear to the wall, and listened intently." Here "to" works even better than "against".
The word "to" has several meanings then. "He pulled the door to." Meaning he gently almost closed the door, but not completely. "To" can suggest a gentle motioning or action, or even "up to" or "close to", without quite touching.
So technically, if used wisely, both can be correct.