ok, time to put all of these half answers together:

first you need to upload it to an image sharing site, imageshack, photobucket and imgur are the most popular, but imgur is the best in my opinion.

doing so if fairly self explanitory, just go to imgur.com, click the "computer" button under the upload images text on the right of the site.

after you do that, and find the image you want to upload from your computer, double click it and it will upload, and bring you to a screen with a bunch of links on the right, find the one with tags around them, and copy it, then you can paste it in any post or private message and it'll appear like this:

ok bye