Short answer: because you took an upside-down photo with your iPhone!
Of course, the devil is always in the details and it's actually a bit more complicated than that. When you look at the photo on your iPhone, it's fine -- so you have no way of knowing it's upside-down. Your iPhone knows what you meant because it stores the orientation in a piece of metadata called an EXIF tag. But when you upload your iPhone photo, the orientation data either doesn't go along for the ride or can't be interpreted and applied by the website. The same thing can happen if you e-mail it.
The bottom line is that this is a known issue with iPhones, and if you want to avoid it, you need to take your photos right-side up -- i.e., with the volume buttons pointing down. Another solution is to rotate the image on your home computer before uploading it.
For further reading on this topic, try the following:
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