It isn't so much a "live wallpaper" as it is a slideshow that changes with the weather conditions. When it's cloudy, your wallpaper will show a cloudy landscape photo and when it's clear, it'll show a clear weather photo instead. You can even set it to stop after your battery reaches a certain level, if you'd rather not spend it on the slideshow.
Check out the entry below at the Tasker Wiki, which contains step-by-step instructions and a set of images to get you started. Of course, you can always find your own images on the net if you don't like them, or even take your own.
Weather Dependent Background Image [Tasker Wiki, thanks Graham]