Rather than using a map, just have the user upload a picture.
You're killing two birds with one stone. First, the user is being shown something to confirm that this is indeed the site they think it is (think: sitekey or the like). Second, they can pick some incredibly detailed point without all the hassle of licensing someone else's data.
All that, and this is still a pretty stupid idea. You have all the same problems with password: users don't want a long one, users want to pick the same one for multiple sites, users tell the wrong people who their passwords are (though, now with more difficult language). All that, plus it's now multiple clicks, pans and zooms to enter your "password", and if the satellite data updates you're screwed.