JSFiddle - React, Tailwind, and code Playground

HTML

<div class="full">
    <div class="half">
        <label>FEFCO: </label><input type="text" name="FEFCO" id="FEFCO">
    </div>
    
    <div class="half">
        <ul>
           <li><img src="http://globe-views.com/dcim/dreams/box/box-02.jpg" data-size="0300">0300<li>
           <li><img src="http://globe-views.com/dcim/dreams/box/box-04.jpg" data-size="0302">0302<li>
        <ul>
    </div>
</div>
    <hr>
<div class="full" id="select">
    <div class="half">
        <label>FEFCO: </label>
        <select type="text" name="FEFCO" id="FEFCO2">
            <option value="">Vyberte</option>
            <option value="0300">0300</option>
            <option value="0302">0302</option>
        </select>
    </div>
    
    <div class="half" id="two">
        <ul>
           <li id="0300"><img src="http://globe-views.com/dcim/dreams/box/box-02.jpg" >0300<li>
           <li id="0302"><img src="http://globe-views.com/dcim/dreams/box/box-04.jpg" >0302<li>
        <ul>
    </div>
</div>

CSS

.half{width:50%;float:left;}
li{list-style:none;cursor:pointer;}
li img{width:100px;height:auto;}

/* MOŽNOST 2 */
#two li{display:none;}

JavaScript

// MOŽNOST 1
$('li img').on('click', function(){
	var size = $(this).data('size');
    $('#FEFCO').val(size);
});

// MOŽNOST 2
$('#FEFCO2').on('change', function(){
	var size = $(this).val();
    $('#two li').hide();
    $('#'+size).show();
});