Well..the problem is that even if you force all of your outgoing messages to HTML you can't control whether the recipient is going to force it back to RTF. There's a chance that if they sent you a message in RTF that their system doesn't support HTML (either by limitation or by policy).

Since the goal is to have the message received in a certain way it doesn't really get you there to force things on your side.

I guess you could encourage/remind your users to send in HTML format whenever possible (they can change their RTF replies to HTML) but you'll still have to live with some percentage of recipients who just aren't going to see your signature file the way you intend it. Nothing you can do about it, really, short of not using images in the signature.