You have to write it in HTML
The HTML image code is <img src="blah">
Of course, you'd replace "blah" with the image URL. To get an image URL you must upload an image at
http://www.images.freeforumtools.com or any other image uploading website.
Now to get the image to be a link...
The HTML link code is <a href="blah">Anything</a>
This might seem a little confusing to first timers, but you'd replace "blah" with where you want the link to direct to, and you'd replace "anything" with what you want the link to say, or appear as. The "</a>" is just and ending so it knows when to stop making the text a link.
Now that you know these, we have to put them together...
The code will look something like this;
<a href="FORUMLINK"><img src="IMAGEURL"></a>