The product images used for TheMarket, will depend on how you add the images in Shopify.
TheMarket will import images that are specifically assigned to variants (e.g colours or size) OR the main images added to the parent product, but not both.
If you add images to the product as per the screenshot below, to the product but not variants, all images will be imported into the system.
If your product has variants (e.g a t shirt with different colours or patterns) then TheMarket will import the images that are added to the variation but NOT the images added to the main product.
For example see this screenshot which shows the images in Shopify attached to a variant and the images we have imported into TheMarket