One year ago I wrote such BeanShell macros.
/**
* Insert_HTML_Image_Tag.bsh
*/

caret = textArea.getCaretPosition();

parent = new Frame();
fileDialog = new FileDialog(parent, "Please select image...");
// Uncomment following line to browse directory of current buffer
// fileDialog.setDirectory(editPane.getBuffer().getPath());
fileDialog.setVisible(true);

if (fileDialog.getFile() != null) {
   image = new ImageIcon(fileDialog.getDirectory() + fileDialog.getFile());
   width = image.getIconWidth();
   height = image.getIconHeight();
   if (width == -1 || height == -1) {
      Macros.message(view, "Selected file is not image!");
   } else {
      tag = "";
      textArea.setSelectedText(tag);
      textArea.setCaretPosition(caret + tag.length());
   }
}

--
Regards, Skalex.

Hi,

I'm working on a plugin for exact this feature.
But I have stress and will need a couple of weeks to finish it.

Lars

It's completely do-able, though not completely trivial. I think it'd be easy enough to get the asset at the caret using the xml plugin, but you'd need to do a little digging around to know how to determine the sizes of various image formats.