You need to firstly register with an image hosting service site like Photobucket or Imageshack, which is kind of like an online folder you store images on, and upload your images onto that site. When you upload an image to such a site you will be given a url for the image, which is what you type in your reply here using [img] tags.

If things work out, your image will appear like so (my images are hosted on Photobucket which is a good free service for it):