In this case, you can use roxymce without TinyMCE intergrated. Just use with fancybox, iframe, bootstrap modal, colorbox or anythings popup/dialog you want.
type
type of displayed media. Required. Value:image
media
dialog
type of dialog. Required. Supported:fancybox
modal
colorbox
iframe
input
input's ID. Required
In your view file, call roxymce widget
[php]
<?= $form->field($model, 'thumb')->textInput(['id' => 'fieldID'])->label(false) ?>
<a href="<?= \yii\helpers\Url::to([
'/roxymce/default',
'type' => 'image',
'input' => 'fieldID',
'dialog' => 'fancybox',
]) ?>"><i class="fa fa-upload"></i></a>
<script>
$("a").fancybox();
</script>