First thing you need to do is visit this forum thread http://forum.xbmc.org/showthread.php?tid=41650 and download the great fanart pack made by timdog82001 for us its 160mb big so not included in the skin for obvious reasons
Next you need to extract the rar file to a location of your choosing (keeping the directory structure). You will notice that the rar contains a bunch of numbered folders in it, the numbers are what xbmc uses to pick the right condition so don't rename them.
Hopefully if you did it right you should end up with a folder structure like this
Now in the skin go to the weather page and Nav left to get to the side menu. Go into the "Misc Options" section and uncheck "Hide Fanart" you will then need to click on the "Set Fanart Path" button and browse to the main folder you extracted the fanart too. In the case of my example above "Weather-Fanart" (not the sub folders).
Click the OK button in the browse dialog and you should now have Fanart based on the current weather conditions in the weather screen and also the home screen when the weather button is selected