You'd have to upload your image to a hosting service, usually
www.*.com or
www.imageshack.us are used. Once you did so, they will display an address to the image you uploaded.
When you use the attachment function, you can directly upload your images to the forum, without the need of an image address. They will appear at the end of your post or, if specified, can be placed inside your post. For that, click the

icon in the editor after you attached an image. (An [attach] tag will be inserted in your post, representing the file you attached.)