Having a strong self-image is tough. You will not find any single person who doesn't have at least a point in their life when they thought they are not good enough. Because we put ourselves in continuous competition, comparison, and whatnot!
There's no need to say we need to work on our self-esteem to improve our self-image. But this seems very vague. Because it doesn't give us any particular guideline from where we can start changing ourselves. So let's talk about how can we clear up the things we need to be done to feel 'more valued' and have a strong self-image...
Feeling 'less' doesn't auto grow. It comes from our surroundings. There's a high chance that you surround yourselves with judgemental people, you go through toxic competition and you have to be with critical/negative minded people all the time. All these reflect on what you think and feel about yourself. So identifying and finding ways to deal with them in a better way is the first thing we need to do.
Not only thinking positive will always help you. You have to increase your physical activity, connect with more people to boost your social energy, and many more to feel good about yourself.
Never forget to watch out if you are putting yourself into a comparison trap; if your social environment is unhealthy etc. Most importantly you need to cut off from all the negativity and save a place for yourself where you can practice self-love, respect, mindfulness to reflect a better self-image.
I think the 'understanding' is more important than anything here. If you realize what you are into, it will be easy for you to find a way out!