Other than an avatar FF doesn't host images, the way you post an image is to hotlink it. It's worth pointing out that a few sites object to this because they get charged for the bandwidth used, so you might get a stroppy message instead of the image you wanted. Most sites tolerate it though.
The way I do it using Firefox is to right click the image and select "Copy Image Location". Then in your post here click the Img button and paste the link between the square brackets, so you end up with something that looks like this:
[img]http://www.sitename.com/image.jpg[/img]
