You're best using Logician's template conditionals for this one - an easy hack to install and you could do this without having to worry about changing files when upgrading
Will there be one image for each forum, or one for most of them? If so, you probably want to append the forumid to the image filename, e.g. image_1.jpg, image_2.jpg and then load it accordingly. Otherwise if it's a smattering of images and lots of forums, you might use a switch: