While inches and millimeters are appropriate measurements in the real world the density of computer screens and printing methods call for a different measurement.
Digital images are measured in pixels.
4″ x 6″ inch file for the web that measures 400 pixels by 600 pixels
4″ x 6″ file for printing that measures 1200 pixels by 1800 pixels
The following chart shows us that pixel dimensions are the key measuring system for digital images. The more pixels = more information = larger file size and better quality image
1000kb = 1mb
kb, kilobytes and mb mega bytes is how much information is contained in the file. For a high resolution file suitable for printing its gonna be 5 or 6 mb but for the web it would be around 100 kb.
We try to keep the file size lower for the web so it doesn’t take too long for the image to load on your computer screen. One way the web developer does this is by using jpeg format.
Jpeg is a compressed format. It shrinks the final file size while maintaining the number of pixels. It’s like a painter who is in a hurry and says oh the sky is blue so he pulls out a large brush and paints in a blue sky. The jpeg formatting fudges some of the pixels making for a smaller file which can be emailed or downloaded but destroying the subtleties in the image.
It is ok to save a copy of a file as jpeg if it has been sized correctly for its final place on the web, however if you send a jpeg to a site for display and they decide they want it a bit larger or smaller it is like making a recording of a recording the quality deteriorates drastically.
For printing we use a non compressed file such as a tiff or pict file. The file size is usually 300ppi so looking at the chart 4″ 6′ inch image for a printed book should be 1,200 pixels by 1,800 pixels. That is the key measurement in this scenario, how many pixels across, how many down, however printers request stuff in different ways so we can use the chart or let the software do the work.