HTML5 geolocalization is probably not the best idea here, because it requires user to accept the tracking (and it can be not possible in some email clients other than web browsers or be simply blocked by them)
In this case it would be probably better to use some kind of GeoIP database and use Google Maps API to generate static image based on coordinates https://developers.google.com/maps/documentation/imageapis/
Note, that most email clients block external images by default.