I'm already aware of that, but thanks for the tip anyway..
I just don't want to "flood" the forum with ads, since we don't have many members yet.
I think that the top placement looks rather nice (it's "supposed" to be an add there), and there's also a large banner visible under the first post in each thread visible to guests only.
But I'll probably add a large banner at the bottom later when we get some more members.