A product thumbnail is a picture you upload to Ecwid control panel → Catalog → Products → the product in question → General → Product image. This image is displayed as the product preview, when your customers open a category the product belongs to, and this image is displayed in the product details page. The product image, displayed on the product details page, and the product thumbnail are the same in Ecwid, and unfortunately there is no setting to upload different images for them.
But there is a workaround. If you would like to have different images for a product preview and the detailed view, you need to add the image that you would like to be displayed on the product details page to the product description (Ecwid control panel → Catalog → Products → click on the product you would like to change the detailed image for -> General -> Description -> "Insert/edit imabe" icon) and hide the original product thumbnail on the product details page.
To hide a product thumbnail on some product details page please apply the following rule to the end of your custom CSS theme (Ecwid control panel -> ‘System settings’ tab -> Design -> CSS themes):
click "Save (Ctrl+S)".