You can save a single frame of any movie that you shot on your Nikon D7100 as a regular image file. Here’s how to save a single frame as a still image:
Begin playing your movie.
When you reach the frame you want to capture, press the Multi Selector down to pause playback.
Press the i button to bring up the Edit Movie screen.
Choose Save Selected Frame and press OK.
The frame appears on the monitor.
Press the Multi Selector up.
On the confirmation screen that appears, select Yes and press OK.
Your frame is saved as a JPEG photo.
Remember a few things about pictures you create this way:
When you view the image, it’s marked with a little scissors icon in the upper-left corner.
The resolution of the picture depends on the resolution of the movie: for example, if the movie resolution is 1920 x 1080, your picture resolution is 1920 x 1080.
You can’t apply editing features from the Retouch menu to the file, and you also can’t view all the shooting data that's normally associated with a JPEG picture.