Falling snow (DuplicateSprite)
The purpose is to show snow falling over a background image.
We 'll use a basic sprite for a flake and Duplicate Sprite for the others. We 'll use scripts to realize effects.
I suppose you know well how to create objects, thus I won't insist on.
⇒ animation ⇐
At first, Download background picture. 
Creating Object
1 - Set up the movie sizes 400x260 (any background color fits). Frame Rate 35.
2 - Insérez image 304.jpg. (menu Insert / Image)
admire my driving on "Alpe d'Huez" Ice circuit .... wearing snowboots 
3 - You create now a very simple snow flake (white of course).
Draw a shape with tool Pencil (use Zoom as well). Name it flocon
4 - Click Modify / Convert / Convert to Sprite. You'll get :
Scripts
In Outline, select Scene_1. Enter script :
In Outline, select le sprite. Enter script :
Don't forget to export :
It's up to you modifying parameters and see what happens...