For contenttype "text/html": If you don't use the setPage method, you need to set base where JEditorPane look for pages/files. As i remember the image is not displayed even with absolute path.

Try this:

  java.net.URL baseUrl = null;
  try {
    baseUrl = new java.io.File(".").getCanonicalFile().toURL();
    HTMLDocument hdoc = (HTMLDocument)yourEditorPane.getDocument();
    hdoc.setBase(baseUrl);
  } catch ( IOException exc ) {
    //log.error(exc);
  }

If you don't use HTML here, i can't help without looking at your code.

Regards, Mike