Check this link:
http://wiki.wunderground.com/index.php/API_-_XML#Forecast_Conditions_Icon_Names
It shows the conditions and the name of the icon that goes with each one. They should all be GIFs, although you could perhaps modify the functions.php code to make them another format.
Please grab the latest dev build that has a fix for this bug from here:
https://sourceforge.net/projects/wincache/files/development/
I’m reporting success with this latest build. Once I installed this, I updated an old plugin with no problems. Thank you for your support.
