Appending a string to a tinyMCE instance

Question

Hi, I need to append a string (an img tag) to a tinyMCE editor instance.

I was doing it like this:

$("#textboxhiddenbytinymce").append( string );
tinyMCE.triggerSave();

with no luck. I was wondering if:

There is a direct method for adding an arbitrary string (I need complete control over it) to an instance of tinyMCE I understand that the way I'm doing it is wrong since tinyMCE works indipendently to the "original" textarea.

Thank you very much!