I am using following codes to fetch the conent of tinymce.
tinymce_content=tinyMCE.get(\'txt_area_id\').getContent();
updated_content=tinymce_content+\"
This will insert an image node at the selected spot (cursor position) in a tinymce editor
var ed = tinyMCE.get('txt_area_id'); // get editor instance
var range = ed.selection.getRng(); // get range
var newNode = ed.getDoc().createElement ( "img" ); // create img node
newNode.src="sample.jpg"; // add src attribute
range.insertNode(newNode); // insert Node