It's a while since I did this but I think you need to create a movie clip in the library with the bitmap inside it, then export that for ActionScript, then add that as the linkage in the tag.
So, if your movieClip is exported as 'myImage_mc", your html will be:
<img src="myImage_mc" width="100" height ="100"/>
Update.
To clarify, here's my symbol in the library:

Here's my actionscript:
import flash.text.TextField;
var textField:TextField = new TextField();
textField.htmlText = "<p>HKP</p><img src='HKP'/>";
textField.x = textField.y = 100;
stage.addChild(textField);
And here's the result (which admittedly needs a bit of tweaking):

Note: this doesn't seem to work if the img is the only tag in the field. You have to add some text, even if it is not visible. An empty P won't work, so either of these will fail:
textField.htmlText = "<img src='HKP'/>";
textField.htmlText = "<p></p><img src='HKP'/>";
... but this works:
textField.htmlText = "<p> </p><img src='HKP'/>";
... which is pretty much a classic Adobe gotcha ;)