If they're on your computer, first make them an attachment (there's a little paper clip icon that will do that). Then they will appear as little thumbnails in your thread that you can click on. If you'd like to have them in your thread as full size pictures, go on and click on them and there's your URL... you could go back and edit your post and insert them now as a URL.
Thank you - 2015 Race Sponsors (click below)!!