The thumbnail is the picture a shopper sees before any other, at a fraction of the size of the product page. Marketplaces generate it from the first image, and many crop that image to a fixed shape on the way, so the edges of the original may not appear.
An image that works as a thumbnail shows one product, large in the frame, on a calm background, with a clear outline. Fine detail, small text and a wide scene vanish at this size. Leaving a margin around the product protects it from the crop; a product that touches the edge of the photo is the first to lose a part.
The quickest test is to view the first image at the size of a search result. If the product and its colour can still be recognised, the thumbnail does its job. Some marketplaces set rules for the thumbnail that are stricter than those for the rest of the gallery.