Every time I read articles or tutorials about saving images for Web pages or sending through e-mail they always mention saving the images with a resolution of 72dpi or Dots Per Inch. The interesting fact about this suggestion is that a computer monitor doesn’t deal with dots or inches, but rather with pixels.
The resolution of a monitor is indicated as two numbers: [number of pixels for width]x[number of pixels for height]. For example, resolutions are indicated as 800×600 or 1024×768. You notice no mention of dots or inches because a monitor only cares about the number of pixels. This means that when creating images for viewing on a monitor it is important to pay close attention to the size of the file in pixels.
Let’s see an example of how this works. Below you will see two images from the same image. The top image has a print resolution of 300dpi, while the bottom one has a resolution of 10dpi. You will notice that when displayed on the monitor, they are identical. With regards to monitors, both are the same size: 225×300.
A 225×300 pixel image will always be displayed as 225 by 300 pixels regardless of the screen size in inches.
Summary
Unlike printing, DPI as no affect on how an image is displayed on a computer monitor. When resizing an image for Web sites or e-mails it is important to resize the image based on the number of pixels, and not DPI.