How do you post a pic that is downloaded on your computer?

You have to upload it to an image hosting site like imgur, then paste the direct like to the img field.

You dont have to use one of those obnoxious sites these dudes recommended.

Easiest way:
post it to a private album on your facebook page. Then, right click the picture in your facebook album and click "copy image location". Then post on here and when you click the "img" link next to the post box, paste the link.