When you use configurable products in your Magento store there is an option to choose what image will be displayed when product is added to the cart. By default, Magento will use the configurable product`s thumbnail image.
To show the simple product`s thumbnail image rather than the configurable product`s thumbnail image in your Magento shop cart please go to System >> Configuration, then in the left menu click the Checkout tab. When the page loads, open the Shopping Cart box, and in that box you’ll see the “Configurable Product Image” option. Select “Product Thumbnail Itself” option.
Be sure that you have uploaded the images for all simple products and to select an image which will be used as a thumbnail. Otherwise, it might happen that Magento will use a placeholder image or configurable product`s image as a fall back.