Weather Icon images won't display

I’m trying to use the Weather Icon plugin. I’ve downloaded the images and uploaded them to the site. I placed the png images into the “weather” folder inside the images directory. I’ve added the following code to my template:

<div id="weather">
{exp:weathericon station="KGRR"
heading="" 
icon_extension=".png"
icon_location="{site_url}/images/weather"
display_icon="yes"  
display_temperature="no"
display_conditions:"no"
display_clouds="no"
}

Everything displays except for the images. Any ideas?!