your textfield must be html enabled and multiline. you can then use an img tag with its src attribute pointing to an image file that's loaded or to a movieclip's linkage id:
tf.htmlText="<img src='linkage id'>";
The html text works, but the smiley is inserted below the text, and the alignment becomes bad?
aligning images in a flash textfield is part art, part trail and error. but almost alignment can be done if you're creative.