I am trying to position the lawnmower ad in the upper RHS of the header of this site (it is the Exmark ad):

http://www.michigan-sportsman.com/exmark_index.html

I have the div for this .gif overlay image in the correct table and am trying to use relative positioning with this style:

#apDiv1 {
height:100px;
left:700px;
position:relative;
top:120px;
width:300px;
z-index:1;
}

The problem is that this adds extra white space at the top of the page before the header that I can't seem to get rid of or figure out why it is happening.

First, get rid of the tables for layout, they are not necessary and they're just bad practice.

Along the way, get rid of the deprecated <center> tags and learn to use CSS to center your site - look in our stickies for help with that.

It likely isn't necessary to use positioning for the ad in the first place is you get your layout out of tables.