Was working on a project and had problems with getting the icon set for Wunderground weather forecast plugin by Zack Katz. Thank you Zack Katz for the plugin! I love it!
While the updated plugin has the icons showing properly for the simple [ forecast ] shortcode, using multiple shortcodes somehow still created problems.
I have edited the plugin file to use the icons stored on your wordpress install (in img folder inside the plugin folder). I am using the Incredible set of icons (only that set will show irrespective of what you choose), or you can change the set of files for your own icons (make sure to keep same names).
Download the zip folder here.