Carefully tagged images can be a sly and somewhat understated way of getting traffic in through the back door. However, a discussion at WebMasterWorld, and commented upon initially by SERoundTable seems to indicate that there may be some Google ranking and indexing changes afoot for images.
The most recent Google image update was seen in early October, however image updates are often not picked up by webmasters as many do not actively track the traffic coming in from images.