"Can you please explain why most of the pictures I took with my D300s are 3-5 MP? What setting that I missed? D300 is a 12 MP camera. Thanks"
Since you reference a range of "3-5" I *believe* what you are looking at are the JPEG file sizes in megabytes. The *pixel* size and quality level of your images is controlled by the "Qual" button (also available through the menus). The file size in bytes (or kilobytes KB, or megabytes MB) and the image size in pixels are two entirely separate things. Using a higher or lower compression ratio, the JPEG file sizes can change considerably while the image pixel dimensions remain the same.