First some basics- if you get this, the rest is a walk in the park

A digital image consists of thousands, or millions of tiny little squares (usually squares) each of which consists of a solid color, like the example here.
Notice that the dimensions of this picture are 8x6. This is a very low resolution picture, of course, consisting of 48 squares, or pixels. Each of these pixels requires 3 pieces of information (numbers) to accurately describe the color of each pixel. This image therefore requires 144 pieces of information to describe the whole picture. Let's call a piece of information a "byte"

So what do we have here?

An image with pixel dimensions of 8x6
An image with total number of pixels = 48
An image completely described using 144 bytes of information.


Notice these Important things:
>The image description has no mention "inches" or or "dots".

Remember digital images, for practical purposes, do NOT have anything except pixel dimensions.

( well, for you purists, they also have bit depth and a couple other things, but they DON'T have inches.There is often confusion over this point, so don't forget this idea!) But the biggest confusion that abounds is when people use the word "dots" to mean "pixels". If somebody says "dots per inch" (unless they're speaking of printer output), they really mean "pixels per inch" so just cut them some slack. And if they're talking about a digital image and using the word "inch", well, they're really making no sense at all, because, as you've seen, images don't have anything to do with inches until you decide to print it.

If we were to print the above image to a size of 1" x 3/4", like a postage stamp, we would be printing it at 8 ppi (pixels per inch)....or as some would say 8 dpi (dots per inch) – which, you can see is just a confusing word because printers actually DO have dots. Please try to not say dots as it just muddies the digital waters.
If we were to print the above image to a size of 8" x 6", we would be printing it at 1 pixel per inch ( each pixel would be a one inch square.

IMAGES DISPLAYED ON THE COMPUTER SCREEN

REALITY CHECK: Now, I had to do some tricky stuff to get you to see that enlarged view of an image that's 8x6 pixels. In the real world, this image would be miniscule on your computer screen. Take a good close look at your computer screen and you will see lots of small dots ( there's that word again) or "screen pixels" - each displays a single color by combining 3 colors of light. There are usually about 100 screen pixels to the inch on a computer screen so a square that is 100x100 screen pixels would be a 1" square. When you display an image on the computer screen, each image pixel will normally occupy ONE screen pixel. So the 8x6 (image pixels) image above will normally show on a computer screen as an image that is 8/100"x 6/100" (very small!) But if you want to display an image on the screen that measures, say, 5"x4", you'd better make sure your image has pixel dimensions of 500 x 400. This is easy to accomplish using programs like Photoshop or PS Elements or any photo editing program. You can also adjust settings on your camera to give you the proper pixel dimensions right from the start if you know how you will be using the image. MORE ON HOW TO DO THIS LATER!

To summarize this last paragraph: One image pixel will occupy one screen pixel when displayed on a computer screen.

(and for you purists, this is not always true, e.g. when using editing programs you and enlarge and shrink images at will. But when you post the image to a website or send in email, it will display at a 1:1 relationship between image pixels to screen pixels.)The actual dimensions of the image size will be different if the screen settings are different than 100 screen pixel per inch, but this number is a good guess for most computers.

THE BOTTOM LINE - SIZING IMAGES FOR THE COMPUTER, WEB, EMAIL---

Adjust your image so the pixel dimensions are 100 times the desired screen dimensions. For example, if you want an image to be 6-1/2" wide on the screen, change the image so that the pixel dimension of the width is 650 pixels. 650 image pixels will occupy 650 screen pixels and the image will display at 6.5" wide. This requires CHANGING the contents of the image - the image started out most likely with a different pixel width. The image needs to be "RESAMPLED", or converted to a different number of pixels from its original configuration. This is a "destructive" process, so always save the original because resampling involves reinventing the image, not always for the better. AND DON'T BE THINKING ABOUT PIXELS PER INCH, OR DOTS PER INCH until you decide to make a print. Printed pictures have "pixels per inch", digital images do not. They only have PIXEL DIMENSIONS.

This website, for example, requires that all images have a maximum pixel dimension of 640 pixels - meaning that neither the width nor the height can exceed 640 pixels. Consequently you will never see an image on here that is larger than 6.4" on either dimension.