The iPhone allows you not only to store and view pictures you've taken using the device's built-in camera, but also to transfer images from a folder on a computer to the device via iTunes. In addition, it's possible to view pictures within Web-based mobile applications such as Facebook and Flickr for iPhone. The reason that underlies difficulty in viewing pictures depends on from which interface you view the pictures.
Camera Roll
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If pictures within the iPhone's Camera Roll -- in other words, ones you have previously taken using either of the iPhone's built-in cameras -- appear black, your phone may be experiencing a problem retrieving the images from its flash drive. Restart the iPhone by holding down its "Home" and "Lock" buttons at the same time to attempt remedying this error.
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Transferred Pictures
If transferred pictures are the ones that appear black on the iPhone screen, it's possible that the pictures transferred incompletely or incorrectly. To troubleshoot black pictures you have transferred from your computer, navigate to the folder where you store images you transfer onto the iPhone, then drag the "iPod Photo Cache" folder into the trash. Connect the iPhone to the computer, launch iTunes if it doesn't open automatically and wait for iTunes to re-transfer all pictures onto the device, making sure not to disconnect until iTunes informs you the process is complete.
Mobile Applications
Mobile applications Facebook and Flickr for iPhone allow you to view pictures you've uploaded to your accounts via the iPhone interface. If your Internet connection is slow, the images appears black until they load fully, which can take several seconds or even minutes. If the Internet connection doesn't work at all, the images only appear black until connection is restored.
Newly Taken Photos
In other instances, photos may appear black immediately after you take them. If you press the "Camera" button to snap a photo and a black image appears on the screen, it means that you obscured the iPhone camera lens or attempted to take the photo in low-light settings and without flash.