Start with a dark background and draw some white "snowflakes" (they can be
simple dots, six-pointed 'stars' and even symbols from the Wingdings2 font in
several different sizes for perspective.
Animate them to move from above the top to various points on the slide where
you want them to disappear/melt (disappear by dimming after the animation or
let the larger ones (closer in view) pass below the bottom. Set them to
repeat until end of slide.
Once you've got several that move the way you want, just copy and paste them
in various places. Adjust the timing and speed to make it look more random.
You can even add a rotation and spin to some of them.
Make as many copies of the slide as you'll need, then simply paste your
pictures and move them to the back.
Have fun!
