this is not completly done, but it works, i used some images from kde4 weather wallpaper plugin so please tell me if there is a problem with that (legally?), also the pictures are changable, just put what ever images you like in "~/.conky/weather-wallpaper" according to the script or just replace the exsisting images, hopfully there will be a nature theme, city theme etc... please vote and give me comments if somthing is wrong.

* this is weather.pl from:
http://ubuntuforums.org/showthread.php?t=666842
modified, added the option 'wwall' to enable wallpaper change according to weather & time. (similar to the KDE4 plugin).

* you can use this as part of conky:
"${execi 3600 ~/.conky/scripts/weather.pl ZIP C/F wwall}" - to have other features of the script with conky you need to have "weather" font, just act according to the ubuntu thread. or, you can add the script to cron or so and run it standalone:
perl ~/.conky/scripts/weather.pl ZIP C/F wwall

* if you dont know your zip, just go to weather.com, type in your city and get it from the URL:
http://www.weather.com/outlook/travel/businesstraveler/local/ASXX0112?lswe=Sydney,%20AUSTRALIA&lwsa=Weather36HourBusinessTravelerCommand&from=searchbox_typeahead

* the images are from KDE4 regular and weather wallpapes, and also from google/devianart etc...

* the images need to be located at "~/.conky/weather-wallpaper", if you want to change that, just edit it in the beginning of the script.

NOTE: this will also change the gnome lock screen wallpaper, you need to have sudo (w/o passwd) for that, if you dont want that you will need to disable it in the script - this is stupid, i will probebly find a better way then sudo for this