Since you already use CSS on your pages you could also remove the margins there.
This is what one of mine looks like.
body { color: white; font-size: 11px; font-family: Verdana, Arial, Helvetica, san-serif; background-color: black; background-image: url("backgound.jpg"); margin: 0; padding: 0
Either works fine.
Putting your ads inside of a one cell table with its width set and then nesting it inside of the cell of the table that is holding your pics helps in keeping the text from pushing the table too wide.
This is how I do it anyway.
I don't give the table or its cells a width and let the pics determine that tables width.
This is also why I use a nested table to hold my ad.