#1. "RE: High Def photography" In response to In response to 0 Sun 27-Jan-13 05:06 PM by coolmom42
Menu button--shooting menu (camera icon)---image quality. Then hit the OK button on the right, and it will show you the image quality options. Choose JPEG fine, then OK again. That will give you the highest resolution JPEG possible. Then scroll down to image size, hit the OK button, and choose LARGE.
Those settings will get you a 4602 x 3012 pixel image.
Some people will say you need 300 pixels per inch for an acceptable image. Others will tell you 100 pixels per inch is adequate. What you need depends on what you find acceptable. I try to go no lower than 150 pixels per inch. The largest image I've ever had printed from my D3100 was 16 x 20, made from a slightly cropped JPG.
The printed user's manual that came with your D3100 is not the full version. Here is where you can download the full version: