Picture size and Resolution explained

Pixel dimensions

The number of pixels along the height and width of an image.

The display size of an image on screen is determined by the pixel dimensions of the image plus the size and setting of the monitor.

For example, a 15 inch monitor typically displays 800 pixels horizontally and 600 vertically. An image with dimensions of 800 pixels x 600 pixels would fill this small screen. On a larger monitor with an 800 x 600 setting  the same image would still fill the screen but each pixel would appear larger. Changing the setting of this larger monitor to 1024 x 768, pixels would display the image at a smaller size, occupying only a part of the screen.

Image Resolution

The number of pixels displayed per unit of printed length in an image, usually measured in pixels per inch(ppi).In photoshop, you can change the resolution of an image; in image ready, the resolution of an image is always 72ppi.

When printed, an image of high resolution contains more and therefore smaller pixels than an image with a low resolution. For instance, a 1x1 inch image with a resolution of 72ppi contains a total of 5184 pixels (72 pixels wide x 72 pixels high = 5184). The same 1 x 1 inch image with a resolution of 300 ppi contains a total of 90,000 pixels.

Using too high a resolution increases the file size and slows the printing of an image.; further more, the device will be unable to reproduce extra detail provided by high resolution.

Monitor Resolution

The number of pixels or dots displayed per unit length on the monitor, usually measured in dots per inch (dpi). Monitor resolution depends on the size of monitor plus its pixel setting. 

understanding monitor resolution helps explain why display size of an image on screen often differs from its printed size. Image pixels are translated directly into monitor pixels. This means that when image resolution higher than monitor resolution the image appears larger on screen than its specified print dimensions. For example, when you display a 1 x 1 inch 144ppi image on a 72dpi monitor it appears in a 2 x 2 inch area on screen. Because the monitor can display only 72 pixels per inch. It needs 2 inches to display the 144pixels that make up one edge of the image.