<script src="//cdn.tinymce.com/4/tinymce.min.js"></script>
<script>
tinymce.init({
    file_browser_callback :
    function(field_name, url, type, win){
        var filebrowser = "yourfile.php?a=param";
        filebrowser += (filebrowser.indexOf("?") < 0) ? "?type=" + type : "&type=" + type;
        tinymce.activeEditor.windowManager.open({
        title : "Insert File",
        width : 700,
        height : 400,
        url : filebrowser
        }, {
        window : win,
        input : field_name
        });
        return false;
    }
}
</script>

// == yourfile.php ==

<script src="https://code.jquery.com/jquery-2.2.4.min.js" integrity="sha256-BbhdlvQf/xTY9gja0Dq3HiwQF8LaCRTXxZKRutelT44=" crossorigin="anonymous"></script>
<script>
$(document).on("click",".file",function(){
  item_url = $(this).data("src");
  var args = top.tinymce.activeEditor.windowManager.getParams();
  win = (args.window);
  input = (args.input);
  win.document.getElementById(input).value = item_url;
  top.tinymce.activeEditor.windowManager.close();
});
</script>
<?php
$path = 'yourimagefolder/';
$files = GLOB($path.'*{.jpg,.png}',GLOB_BRACE);
?>
<?php if (!EMPTY($files)): ?>
    <ul style="display: inline-block; width: 100%; box-sizing: border-box; list-style: none; padding: 0;">
    <?php foreach ($files as $file): ?>
        <?php
            $file_name = basename($file);
            $file = str_replace(' ', '%20', $file);
        ?>
        <li class="file" data-src="<?= $path.$file_name; ?>" style="margin: 10px; width: 100px; display: inline-block; height: 100px; float: left; cursor: pointer;">
            <div class="file_image" title="<?= $file_name; ?>" style="display: block; width: 100px; height: 100px; background-image: url(<?= $path.$file_name; ?>); background-position: center center; background-size: cover;"></div>
        </li>
    <?php endforeach; ?>
    </ul>
<?php endif; ?>