I think that your solution is a hack named ad management With this hack you can put a banner at the top on the left, on the right, in a left column or in a right column. Sorry my english, i hope you understand me.
that should do the trick... otherwise you need to fix it in your forumhome template.. place the banner code on top of the forumhome template and align it to the left or the right by putting a <align=left> or <left> </left> code (other codes also can do) there to put it were you want