I use IconFactory (which I wrote for this purpose) (
http://smalltalkhub.com/#!/~peteruhnak/IconFactory or Catalog Browser ), it works in the same way as Pharo icons (base64 representation of the image which is then converted to a Form object).
Catalog browser offers also ExternalIconFamily but I have no knowledge of that. But I remember that someone was working on referencing external files, so maybe this is it.