thanks Heath. I got it working by doing this:
ResponseWriter writer = context.getResponseWriter();
writer.startElement("img", this);
StringBuffer src = new StringBuffer();
... create the URL, starting with a "/" but no context-root
String url = context.getApplication().getViewHandler().getResourceURL(context,
src.toString());
writer.writeURIAttribute("src",
context.getExternalContext().encodeResourceURL(url), null);
writer.endElement("img");
