The script
This allows you to put in the number of snowflakes. Positions are calculated in portions allowing for any sized screen. The Y positions are increased by portion definable in the script every 100ms. The X positions are increased by the sum of two sine expressions (making the snowflakes swing from side to side) with the rate of swing being adjustable in the script. Browser window settings are read at every point, allowing scrolling and resizing to be taken into account. When a snowflake reaches the bottom of the screen it is returned to the top. The image used can be adjusted in the script, allowing any picture you choose.
To download the script(s), see the script license, and check details like browser compatibility, use the links on the navigation panel at the top of this page.
Demonstration
The demo page is held here inside an inline frame: