If you take a picture of a car or house key, could you use that picture to get a copy made? Yes — quite trivially, actually. I have a folder on my laptop that is filled with photos people have taken of their keys and put onto the internet. Every few weeks, I take some idle time and associate one of those keys to an address (lot of Googling, mostly) and then I decode the cuts in the key.
Then, I do nothing else — because I'm a pretty nice dude who is just fascinated by this sort of thing. That said, however, the specific measurements for any common brand of lock can be found online, and, with a little experience, you can hand-file keys in only a few minutes. Just search for "Depth & Space" charts. Those will tell you how far apart to space your cuts and the possible depths you might find cut into that type of key. Note that, while the space and depth will stay consistent across a given brand, it's up to you to figure out the specific depths for your key.
Because you are working from a photograph, you may also need to do a little work to scale and skew the reference image a bit. However, you can figure out the dimensions of other parts of the key — like the total length, height, bow of the key, etc. — so, even if the original image was taken from an odd angle, you can use various known points on the key to normalize your measurements. Compare the depths of the cuts to the depth & space charts — and, boom, you have what we call the "bitting" of the lock.
Then, just take a blank key (you can buy the most common blanks at your local hardware store) and mark out the spacing with a Sharpie. File down at each Sharpie mark until you've reached the right measurements, and you'll have a working key for that lock.
You should think of your keys like you think of your passwords: don't show them off to the world!
A variation on this post was originally posted on Quora. Lead image by Flickr user Chris Cook/Creative Commons.