<img class="opacity"src="https://citaty.info/files/styles/poster/public/characters/144853.jpg?itok=hdpicg0G" alt="">
.opacity {
  opacity:0.3;
  cursor:pointer;
}
.active {
  opacity: 1;
}
let opacity = document.querySelector('.opacity');
opacity.addEventListener('click', function(){
  opacity.classList.toggle('active'); 
})

External CSS

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External JavaScript

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