Image Resolution is best understood as a rectangular grid of picture elements (pixels). Resolution is traditionally set in pixels per inch (ppi) also know as dots per inch (dpi). The resolution setting maps an image’s pixel dimensions to its physical size. With a pixel dimension of 600 x 600 and a resolution of 300 DPI, the image has a physical size of 2” x2”.
The resolution setting is crucial when you want to print. A 1600 pixel x 1200 pixel image printed at 72 ppi, for example, would output at over 22” x 16”. However, at 300 ppi, that same image would output at 5.33” x 4”, or about the same size as a postcard.
For an image to print properly, the image should be saved at 300 ppi at the final printed size.
The pixel dimension of a 4” x 6” image at 300 ppi is: