Setting aside why you would want to (it's to pick up girl's isn't it?) You could have a published PGP key, which is also on your blog card. This of course is not immediate, people would need to email you against and get a response. In fact getting something immediate would be difficult. You could have a hidden photo page bookmarked on your mobile perhaps. Of course how do we know it's a true likeness, or it wasn't hacked up there? In fact, how do I know the person who took my blood pressure at that blogger dinner and ate at our table wasn't some mad imposter? Uh-oh.