Your images are made up of many small squares of color called pixels. The number of pixels across the width and height of the image is called the resolution. An image with 640 pixels across the width and 480 pixels high has a resolution of 640x480. The more pixels in the image file, the higher the resolution. Many cameras offer a choice of resolutions from 640x480 up to 3000x2000 or higher. The larger the file resolution, the fewer images that will fit on your camera memory card. Commonly the size of a digital camera image is proportionate to a 4x5.3 not a 4x6.
Images viewed on a computer monitor are displayed at 72 dots per inch or dpi. Each pixel is converted into a dot of color. An image with 640x480 resolution will be displayed on a monitor as 640/72=8.9 inches by 480/72=6.7 inches, or 8.9 x 6.7 inches in size. Therefore an image with 640x480 resolution is a good size image to view on a monitor, email to a friend or use on a website. Unfortunately, this resolution is too small to make a quality print. Here’s why…
In order to produce a “photo quality” print, we must print at 300 ppi (pixels per inch); have at least 300 pixels for every inch of the print. In order to produce a “photo quality” 4”x6” print we need a file resolution of 6”x 300ppi=1800 pixels across the width and 4”x 300ppi=1200 pixels in height. Therefore we prefer a file resolution of about 1800x1200 or higher for optimum 4x6 prints. This resolution would require a 2.1 megapixel digital camera (1800x1200=2.1 million pixels). A quality 8x10 print would need an image file with a resolution of 3000x2400 or higher. Smaller resolution files can be printed, but the results will be less than optimum.
In a nutshell, multiply your ad size (in inches) by 300 - across and down. Have a look at the example below.
Let's say you have purchased a double full page spread with a background image. The ad measures 16.25" x 10.625"
Multiply each side by 300 and you get a needed resolution of 4875 x 3187 pixels. Yes, that's a big image, but quality printed results rely on it.