The hard fact is that the music business is very tough to get into and neither musical talent nor image alone will get you there, you have to present a finished 'product'. Look at the media phenomenon Pop Idol for example and the thousands of people who were hoping to 'make it' but didn't get through the first round.
Image is of course important; it is part of the way that people judge and regard you in life but image can be changed. We've all seen pictures of pop stars, models and celebrities in magazines looking rather different to how we normally perceive them in the media! Image can be changed and worked on particularly with the help of make-up artists and style advisors.
That is not to say that image isn't a part of your performance, it is part of the 'finished product'. The key to image in performance is self confidence and you will only do that by knowing your material back to front so you don't have to worry about singing or playing the right notes and you can get on and enjoy yourself. When you are performing make sure you are happy with your own image; you are wearing clothes you like, hair looks good, and no lipstick on your teeth etc! That will help you to perform well.
Try to develop your own musical material and develop your own musical style and in doing so you will create your own image: be proud of who you are and most of all, believe in yourself.