I think image is important, and certainly some very bright people don't achieve what they might because they have a poor "image" or self presentation.
However I also think people who have made it up the ladder can have a habit of being overly self-deprecating at times - its as if they choose to forget the long hours they put in, the weekends, the hard work,etc, cos they prefer to think of themselves as naturally charming and bright rather than simply hard working. Its weird but I see it a lot...(although yes, there are those who two minutes after first meeting you are boasting about how early they start work, etc)
Also I don't think image is just about looks or clothes, but also about the way you act. In meetings, for example, do you dominate, or do you sit silently and then summarise at the end? Are you a visionary or do you get things done? Are you warm and approachable or are you a cold but inspiring genius?
All of these are part of your image...
Personally I think if image is more than 30% of what you have to offer you will be found out before you make it to the top.