Open up the Wc3 Viewer and find the icons you wish to use. Save them as .jpg's or .bmp's or something like that. Open the file in any image editing program and scale the image down to a lower size. I use 40x40. Now, here's the difficult part. Go find an image hosting website like ImageShack or Photobucket and submit your icon there. Copy the IMG address, which shouldn't be too hard to find, and paste it into your signature.