Unfortunately, the custom HTMLEditorKit subclass does a lot of other
things as well... and I don't have time to separate the parts that
matter from the parts that don't.  I described the basic steps to
implementing the synchronous image loading, which is the biggest deal.  
Other problems may come from background images and the like.

It would look something like this (entirely from memory, so let me know
if you can't fix the mistakes I'm sure to make):

class SyncEditorKit extends HTMLEditorKit
{
   public ViewFactory getViewFactory()
   {
       final ViewFactory superFactory = super.getViewFactory();
       return new ViewFactory() {
           public View getView(Element e)
           {
               View v = superFactory.getView(e);
               if (v instanceof ImageView)
               {
                   ((ImageView) v).setLoadsSynchronously(true);
               }

               return v;
           }
       };
   }
}

You'd then have to do the following to the JEditorPane:

   editorPane.setEditorKitForContentType(
       "text/html",
       new SyncEditorKit());

Oh, no!  This is actually defined from within a Component subclass.  In
my specific application, the Component will later be drawing the image
to the screen, so that makes sense.  I thought I mentioned that you'd
want to use "new BufferedImage(width, height)" instead.