Resolution  in digital imagery refers to pixels per inch (ppi) or density, similar to how many square feet a particular room in a house might have.  Currently, computer displays are limited to a range of 72 to 96 ppi.  For the web, 72 ppi is the standard.  Increasing the density is practical only for images intended for print and results in large file sizes which in turn result in longer download times. 

Dimensions
The amount of pixels contained in the width and height of an image is referred to as an image’s dimensions.  For instance, an image might be 320 pixels wide by 200 pixels high (commonly expressed as 320x200.)  Image dimension is a key element of web design.  Currently computers have a common group of fixed desktop dimensions ranging from 640x480 to 1280x1024 or higher.  Web sites are usually designed “resolution-safe” meaning to the lowest common denominator, a desktop display at 640 pixels wide by 480 pixels high (640x480).  Within the browser itself a width of 600 pixels is the common limit.  This ensures that everyone viewing web sites with image capable browsers will be able to view your site.

Color Depth (Bit-Depth)
Color depth describes the maximum amount of colors an image can contain.  The number of colors an image contains is dependent on the image’s bit-depth.  Confused?  When users describe an image’s color depth, they might also describe it in terms of bit-depth.  The higher the bit value the more colors the image can contain.

Images are made up of one of four standard bit depths: 1 bit, 8-bit, 24-bit, and 32-bit.  A 1-bit image consists of either black or white, or two colors.  8-bit images contain a maximum of 256 colors or 256 shades of gray.  24-bit images contain a maximum of 16.7 million colors.  A 24-bit image is made up of three 8-bit channels, a Red channel (256 shades of Red), a Blue channel (256) shades of Blue), and a Green channel (256 shades of Green).   32-bit images would contain over a billion colors; however, the final 8 bits of the image are commonly reserved for alpha channel information rather than color.  Alpha channels are covered later.

A 24-bit image consists of three (3) 8-bit channels (256 x 256 x 256), which make up a potential of 16.7 million colors.