You ever watch CSI or NCIS? In one episode there was a missing girl that they needed to find, but all they had to go on was a single photo. They zoomed in on her eye and in the reflection of her eye is the porthole of the boat she was being kept on. A mile away is another boat blocking a roadsign so they digitally move it away so they can see what's behind it.
Of course, they're pushing the limits of fantasy, but there's a lot of truth in it too. I once asked a girl that was selling some furniture to send me a picture. When she did, I looked at the furniture, but also the reflections in the glass where I saw the flash from her camera, and her standing in her underwear below that.
While I never told her that I saw her in her underwear, I've told literally thousands of people since then! Unless you want to be in her position of being immortalized forever in a simple mistake, you should learn a little about photo safety.
Backgrounds and Reflections
Here's a nice photo of your house. No worries right?
Well what about your poor neighbor who was minding her business only for you to post a photo of her in the shower.
How about the fact that your state, street, and house number are visible leading them right to your house? Or that the fancy stuff in your garage gives them good reason to target you.
Here are several other examples where people didn't pay much attention and ended up embarrassing themselves (WARNING! Adult oriented and probably NSFW).
When creating manuals, help guides, or when looking for help, people post pictures of their screen online, but often don't think about the consequences. Take this example I found in a forum where a guy was asking for help trying to figure out why all his shortcuts looked odd. Of course, this exposes the names of his files, programs, and links. Chances are he didn't want anyone to know just how much he loves the girly twilight phenomenon!
Companies don't want hackers to know what operating systems and programs they're using, but a simple screenshot can give that away in seconds. Not only that but many screenshots I've seen have e-mail or calendars open in the background which gives away important dates, names, accounts, etc.
If you can get rid of the sensitive information by cropping out unnecessary parts, do so. Otherwise, you might have to abandon posting the photo at all (unless you're sure there's nothing at all sensitive in the photo).
I already told you the story of the girl who flashed me without meaning to in her photo. What I didn't tell you yet was how common this actually is! So common that they named it Reflectoporn. Here's one example:
Makes you want to check more carefully before uploading huh?
If you take a look at this photo, nothing seems wrong. It's a cute girl catching some sun and not much more. Nothing in the background, nothing in the reflections. But what about the EXIF data?
EXIF is a set of data items that are added to a photo and go everywhere the photo goes (it's part of the file). If you put your name into your computer as the user or whenever software asks you, that data could be loaded into the camera, and thus the photos.
How to know for sure
You can check by right-clicking a photo you've taken and clicking on "Properties" then clicking the "Details" tab. Alternatively if you use any real photo editor (not MS Paint) one of the options should be to view and modify EXIF data. Also if you have an account with a photo sharing site like Flickr, you can also just upload a photo there and click the "this photo was taken with [model of camera]" link to see all available data.
But wait. It gets worse. Did you know that some cameras and a LOT of camera phones (iPhones for example) geotag photos? That means that part of the EXIF data is the GPS coordinates where the photo was taken. That allows nifty features like this Google map overlay with tons of persona photos people uploaded pinned to the location they were taken! Neat right?
Why this is bad
So now you can know where any pictures are taken. Remember that photo of the cute girl up there? Maybe some creep decides he'd like to know where she lives and now, with a click or two, he can. Just put the GPS data into Google maps!
What about our soldiers out protecting us the best they can wherever they are. It's kind of important that people not know where they are or the camp they're in or the boat they're on. As soon as someone takes a candid shot and uploads it, they may give away their position and literally be dead by morning.
Whether you're a relative of the military, a supporter, or military yourself, don't underestimate the dangers of giving your position away!
What to do
Photo editors can remove EXIF data and you can click on individual data items in the Windows property screen and delete them as well (though this takes time). In Windows 7, you can click a link in the properties window labeled "Remove Properties and Personal Information". Another simpler method is to disable geotagging in your camera/phone in the first place!