JSFiddle - React, Tailwind, and code Playground

HTML

<ul id=sets>
    <li>
        <img src="http://c3397992.a.cdn.imgwykop.pl/suhy_AR4ZLZ67Jp,q150.jpg" class='size-button'/>
    </li>
    <li>
        <img src="http://c3397992.a.cdn.imgwykop.pl/suhy_AR4ZLZ67Jp,q150.jpg" class='size-button'/>
    </li>
</ul>

JavaScript

$('.size-button').click(function() {
    console.log($("img", "#sets").index(this));
});