Thank you for your question.
Yes you can change the thumbnails of the widget as you wish.
I would advise you to follow these steps:
– Right click on the widget and from the menu click on Clear Widget Contents
– Add all images to the slider
– Select the widget and click on the blue arrow icon on the right top corner to open the widget settings and check Free Form Thumbnails
– Not you can reposition the thumbnails as you wish.
I hope this helps.