One of the great things about Gmail was that the ads were relevant and unobtrusive. The huge ad for Vistaprint is not relevant and it's really obtrusive. "With Gmail, you'll never see pop-ups or untargeted banner ads. Instead we display text ads and related links you might find useful and interesting." This was one of the top 10 reasons to try Gmail, but it was removed from this page.
A Gmail help page explains that "occasionally we may show a banner ad on the Gmail logout page. This ad is different from other ads in Gmail since it's not related to the content of your Gmail account. Instead, it's the same type of ad that appears on the YouTube homepage."
New York Times reported in May that "Gmail will begin allowing advertisers to use images. For example, an e-mailed offer for a ski package showing a skier on the slopes could be accompanied by an ad on the right side of the screen, showing a competing offer, replete with another skier coming down another slope. The image used in the ad would be static, not animated, and would be used only in cases where the e-mail message itself showed images."
{ Thanks, Gary. }