That's because it's a textbox and not a default placeholder for images. You would have to draw the control yourself and when you encounter the sequence ":)" you wouldn't draw the letters but the image instead. There is some more work to it than simply replacing a text sequence by an image. You are now mixing different types that do not apply.
You could however use an rtf control that already has implementation for this, have a look here:
http://www.codeproject.com/KB/edit/csexrichtextbox.asp[
^]
Good luck!