You are adding images to a product in Store Commander. All images formats are listed except the "thickbox" format.
For this format to be created by Store Commander, it needs to be first associated to products.
To do so, from PrestaShop backoffice > Preferences > Images : make sure "thickbox" format is associated to products.
Once done, run the tool 'Regenerate thumbnails' further down the same page to recreate the format for all your older images.