Here is what i did. In your template.cfg, you might have section for winowds and linux. In the host define section add line as this :
icon_image linux.png (linux.png is the icon file for the linx hosts)
similarly , for your windows do the same
icon_image windows.png (windows.png is the icon file for windows host
Save the template.cfg file
Then, you need to upload those two files in right place and right size. here is the location
PS: check out the other file size in that location then make your icon image as similar to that size.
Hope this helps.