So I can recall that Matt Cutts has said in the past that often Google can determine if something is an ad such as a banner through their algorithm and will not have those links pass pagerank.
So let's suppose I make image called advertisement1.gif with a title and alt tag called "advertisement 1". Would that be enough of a declaration to google that this is an advertisement without having to add the "no follow" tag?
The reason for this is I know I could get more for the ads if this strategy was permitted.