You could try something like this :

1. Select all the images to be included in the popup
2. Click the Popups tab and select WidgetKit from the Popup Type list.
3. Click the Insert Multiple Images button
4. Set your WidgetKit popup options
5. Set a Group name for the popups.
6. Click the Advanced tab and in the Style field add : display:none;
7. Click Insert
8. click the Source Code Editor button.
9. Find the first link with the data-lightbox attribute and remove the style="display:none;" from its child img element.

Hello from Greece,

is there a link with the instructions on how to use the Yootheme widgetkit plugin with a single thumbnail image gallery?

Thank you in advance

What JCE version are you using? MediaBox does not provide a dialog for the editor, are you referring to the Media Manager?