Most pictures allow you to to save them by using your mouse "right  click" then "Save Image As". Name it so it's easy to find on your computer. Now you can attach it to your post just like any other picture.

Another way to capture an image or a copy of your screen is to use a "Screenshot".  A common program to use for this is Fireshot. It works in Internet Explorer, Firefox and Chrome. You can download the free version HERE.