FSM will not host the picture file per se. You need to find another place on the web to post it. Photobucket is a popular one that works well with the FSM forums. Or, if your ISP gives you a personal web site, that will usually work too.
Once it is on the web, you need to copy the address (URL). Photobucket is popular because this is almost automatic. Once the address is on the clipboard, start a message. Then click on the filmstrip icon. A window will open up asking a few questions, including the URL of the image. Paste the URL in here. Or, merely right down the URL of the image site and hand enter it in the window. Pick max sizes- I recommend about 600-700. Then ok the window and finish the post.