Negative messages are all around us. You must choose to rejct them and likewise choose which messages you will believe and make part of your self-image. You can be your own coach, arguing against negative messages with positive messages that describe the healthy you. Keep a written and mental list of words that describe positive characteristics you have and some that you would like to have. Use those words as guides for your behavior which will create the positive image you seek.
Assertive behavior is an important part of a positive self-image.
Being able to express your views and feelings affirms your right to self-determination and says that you regard yourself as an important person. Even things you do that may seem insignificant, is done well, can affirm your specialness and add to your positive self-image. It will be reflected in your feelings and actions. It will do wonders for how you feel about yourself. Eventually, they will affect your self-image.