Lame_Wolf - 3:00 am on Mar 8, 2012 (gmt 0)
My first thoughts on this were... "how wide is your site?"
a big image be placed on the left of leaderboard with lots of whitespace in between
Seeing that a leaderboard advert is 728 pixels wide, how big is big ?
You could have a header/title above both. One saying "Web content" and the other saying "Advertisement"