You have to have it hosted on a server somewhere. I use the pictures section of my Myspace to host all of mine. Once you have it hosted, right click the pic you want in the post then get the address from the properties section that will be one of the options that pop up. Once you have done that, cut or copy the address by highlighting it in right click mode then hitting cut or copy. Once you have done that, go to the section you wish to post in, click the IMG button, paste the address after it and click the IMG button again. There. Now you can post a pic!
The Avatar pic is download-able from your computer to the forum's server, but must be cropped to the right data size in order to work. Once you have set the size right in your computer ( using a picture editing software like the one that comes in Vista Premium), hit browse in the forum's user control panel, pick the picture and set it to Avitar.