Resolution
Images are measured in physical size and resolution. For most uses an image will need to be a specific size. Pictures can be re-sized and their resolution altered for these purposes.
Original image from Canon 10D ( 6.3 megapixel )
Same image but showing the relative size in inches at 72 dpi.
Same image but showing the relative size in inches at 300 dpi.
Resized to 4500x3000 pixels
Resized to 4500x3000 pixels, but showing the relative size in inches at 300 dpi.
