How are product images handled?
When you use a product grid inside of an email trigger, it will pull the product images from the products in question (whether that's products linked to the quote in the case of an abandoned cart email or the order in the case of a post purchase trigger).
The product image will respect simple product visibility, as well as the parent relationship with configurable products. So if you have a child product that isn't visible individually, it will grab the image from the parent product.
If you have multiple product images, note that it will grab the Small image. This is the image that is used in category listings on site as well and it tends to work well within grid layouts: