When you take a photo with your phone, it records your location so that you can plot your photos on a map. This is a handy way to organize photos, especially when you have a lot.
Eons ago, when people still took photos with digital cameras (not phones), you could buy accessories like the Eye-Fi to add this geotagging capability. Eye-Fi used wifi networks around you to figure out where you were (WPS).
If you take a photo in your house and share it on the internet, you have just shared your home address with the world, even if there’s nothing visually in the picture that would identify where you are.
In addition to the visual information that you think of as “the photo”, there is something called EXIF data packed into the photo file. EXIF is where the location info is stored.
If you share to a service like Facebook or Twitter, they may strip out this EXIF data before publishing to your friends and followers. But FB/Twitter still receive the information in the first place.
If you’d like to prevent this information from leaking out, you need to strip the EXIF data from your photos before sharing. Currently, this is a pain in the ass. The trick I use is to take a screenshot of the photo I’m about to share, and then share the screenshot instead.
It’s up to you whether this is something you care enough to do. But now you know. I think privacy is largely impossible to achieve, but there are some easy wins. If enough people care about this kind of thing, iOS and Android could implement a setting that automatically reduces the precision of location data in photos that you’re sharing.