Snow doesn't fall too often, and certainly very rarely in April in Europe. When it does fall it does not usually stay for long. This song is about such a special event, and an opportunity missed, not seen, or not taken - and that once over it will not return, and as time goes by you can no longer see or imagine it. Instead it rains..... Take the opportunity when it comes is the message in the song.
About the picture: This is different from the other pictures which are more cartoon-like. This face is very difficult to define and the idea of it fading over time is easier to understand.