Paul,

 

Try the following code:

 

<Script Language="javascript">
    function callInsertImage() 
    { 
   var editor1 = document.getElementById('<%=Editor1.ClientID%>');
            editor1.FocusDocument(); 
            var editdoc = editor1.GetDocument(); 
            editor1.ExecCommand('new');
            editor1.ExecCommand('ImageGalleryByBrowsing');
            InputURL();
    }   
   
    function InputURL()
    {
  var editor1 = document.getElementById('<%=Editor1.ClientID%>');
        var editdoc = editor1.GetDocument(); 
        var imgs = editdoc.images;
        if(imgs.length>0) 
        { document.getElementById("imageFld").value = imgs[imgs.length-1].src;
            editor1.ExecCommand('new');
            document.getElementById("imageFld").focus();
        } 
        else
        {
   setTimeout(InputURL,500);
        } 
    }      
</script>