I don't think thats how you u/l from you computer. When you post a thread scroll down the page a little and click the attach images button. It will ask you to select the pic to upload, select the ones you want, click upload and wait. Make sure it's not over 250kb. Another way is to use a photohost, like I do. I use photobucket.com and the advantage is you can upload slightly larger files with them.
The way you are trying is how to show a pic from a http link, a website.
hope this helps!