There is nothing to prevent you from adding {sitepath} in the URL, but the image won't display in the editor when you do, as the URL would be invalid. A workaround coul be the following: Create a file called editor.js in media/jce/js and add the following:

(function () {
  tinyMCE.onAddEditor.add(function (mgr, ed) {
    ed.onPreInit.add(function () {
      ed.parser.addAttributeFilter('src', function (nodes, name) {
        var i = nodes.length, node;

        while (i--) {
          node = nodes[i];

          if (node.name != 'img') {
            continue;
          }

          var src = node.attr('src');

          if (!src) {
            continue;
          }

          if (src.indexOf('{sitepath}') !== 0) {
            continue;
          }

          var orig = src.substr(11);

          node.attr('src', orig);
          node.attr('data-mce-src', src);
        }
      });
    });
  });
})();

This will allow the image to display correctly in the editor, and keep the {sitepath} "variable". This will not work however if you update the image using the Image Manager after you have inserted it.