Text and Images
Choosing which to use can play a very big role in you site.
Text is good for inline advertising, making teh advert look embedded in your text, also if you choose one such as a wide sky scraper it will show four different advertisements relating to your site, which gives you a 4x chance of displaying an advert of interest to your viewer.
Image adverts are useful because they are more eyecatching, though they are not much use as inline adverts teh old adage 'an image is worth a thousand words comes' comes to the fore. An image is much better at directly influencing a persons emotions, so if used correctly may help increase clickthroughs.
If you reffer back to the heatmap this would be a good place to point out placement of different kinds of adverts.
A text advert will need to be in a place where a viewer will read the text, whereas an image may capture someones eye wherever it is on the page. Judging by this it may help to place text adverts in the high view areas, and banners in places where the eye does not take in as much detail. in all cases the best way to see what works best is simply to play about with it. Different things may work for your site than what will work for others!