If someone buys a text link from my website, I add the nofollow tag so you know it’s a paid link. Should I do the same for banner images also?
In general, whenever you’re talking about banner ads, the vast majority of those are sold via ad exchanges or various advertising networks which already will do block out bots. They don’t want bots crawling their banner ads. They don’t want bots messing with their impressions, or clicks counts, or anything like that. So if you’re using any sort of standard advertising banner ad package, most of the time those redirects will go through things that are blocked by robots.txt or that somehow are not really crawlable by search engines.
However, if you are directly selling a link yourself and the only difference is: an image versus a text link, if someone paid for it specifically, then yes – I would put a nofollow on the link, even if it’s an image link.
Again, for most cases, Google handles the typical banner ad and all those sorts of stuff very well. So that they don’t float page rank and things are handled appropriately. If that’s the case, then I wouldn’t worry about that situation.
by Matt Cutts - Google's Head of Search Quality Team