JSFiddle - React, Tailwind, and code Playground

HTML

<div class='popupbox' id='showimage1'>
    <div class='popupbox_image'>
        <img src='http://yastatic.net/www-skins/1.57/skins/big/piter/day/piter-logo-ru.png' />
        <div class='navigat'> <a href='#' id='link_back1' class='back_g' style='padding-left:0px;'>link_back1</a>
 <a href='#' id='link_next1' class='next_g' style='padding-right:0px;'>link_next1</a>

        </div>
    </div>
</div>
<div class='popupbox' id='showimage2'>
    <div class='popupbox_image'>
        <img src='http://yastatic.net/www-skins/1.57/skins/big/piter/day/piter-logo-ru.png' width='400' />
        <div class='navigat'> <a href='#' id='link_back2' class='back_g' style='padding-left:0px;'>link_back2</a>
 <a href='#' id='link_next2' class='next_g' style='padding-right:0px;'>link_next2</a>

        </div>
    </div>
</div>

JavaScript

var containers = document.getElementsByClassName("popupbox_image");
var shh = containers.length;
for (var i = 0; i < shh; i++) {
    containers[i].children[0].onload = function() {
        var w = Math.ceil(this.offsetWidth/3);
        var navigat = this.parentNode.children[1];
        navigat.children[0].style.paddingLeft=w+"px";
        navigat.children[1].style.paddingRight=w+"px";
    }
}