Pixels and Image Size
A digital image is a computer file that contains graphical information instead of text or a program. Pixels are the basic building blocks of all digital images. Pixels are small adjoining squares in a matrix across the length and width of your digital image. They are so small that you don’t see the actual pixels when the image is on your computer monitor. Pixels are monochromatic. Each pixel is a single solid color that is blended from some combination of the 3 primary colors of Red, Green, and Blue. So, every pixel has a RED component, a GREEN component and and BLUE component. The physical dimensions of a digital image, are measured in pixels and commonly called pixel or image resolution. Pixels are scalable to different physical sizes on your computer monitor or on a photo print. However, all of the pixels in any particular digital image are the same size. Pixels as represented in a printed photo become round slightly overlapping dots.
 

This Picture is 260 pixels in width and 175 pixels in height. It has an image resolution of 175 by 260 pixels or 45 kilopixels since the length times the width equals 45,500 pixels. There are TWO resolutions for any digital image. The first and most important is IMAGE resolution which is the actual pixel dimensions. The second is PRINT or DOCUMENT resolution, also known as OUTPUT resolution . Print/output resolution scales the existing pixel dimensions to a requested print or document size. We'll discuss print/output resolution in more detail later.

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