#1. "RE: D5100 malfunctioning - black rectangle appears in photos" In response to In response to 0
Welcome to Nikonians! Sorry to hear of your problem. Please post a sample image or two to your User Gallery with the Exif data intact. We can then have a better idea of what is hapening.
What exposure mode were you using?
Were you using Flash?
From the information you provided, I suspect that that you were using flash and the shutter speed was set significantly faster than the maximum sync speed of 1/200th sec. Good Luck and Enjoy your Nikons!