Re: Ad placement
If you use adplanner, you can choose some placements for some sites. Check it out at google.com/adplanner. It has many options for you to select placements according to several criteria.
But for websites where there are not so many details, I don't think you can.
What you can also do is use only a (or several) certain image sizes, such as 728 x 90, for instance, or 128 x 600 (if I'm not mistaken, I'm quoting from memory).
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