Your Nikon D3100 can shoot images at a much larger size than you need for sharing them online — photos on the web don’t have to be high-resolution to be viewed clearly. As for e-mails, if your recipient just wants to see (or post) the photos and isn’t planning to print them, they can be low-res — plus, you can fit more low-res photos in one e-mail. Your D3100 can resize images to fit your purpose before you even transfer them from the camera. If you want to create small copies of several photos on your memory card, you can save time by using the following alternative in-camera resizing process. (These steps work with both JPEG and Raw files.)
Press the Menu button and then display the Retouch menu.
Select the Small Picture option and press OK.
Select Choose Size.
Highlight the size you want to use and press OK.
You return to the Small Picture screen.
Choose Select Image and press OK to display thumbnails of all your pictures.
Move the yellow highlight box over a thumbnail and press the Zoom Out button to “tag” the photo for copying.
You see a little icon in the top-right corner of the thumbnail; press the button again to remove the tag if you change your mind.
After selecting all your pictures, press OK to display the copy-confirmation screen.
Highlight Yes and press OK once more to wrap things up.