JSFiddle - React, Tailwind, and code Playground

HTML

<img src="http://upload.wikimedia.org/wikipedia/commons/thumb/1/15/Colossoma_macropomum_01.jpg/800px-Colossoma_macropomum_01.jpg" />

CSS

img {
    opacity: 0.5;
    filter:alpha(opacity=40);
}
img:hover {
    opacity: 1.0;
    filter:alpha(opacity=100);
}