If you include a link as part of your Pinterest post (e.g. a web URL), it will use this as the website (instead of s3.amazonaws.com), even if the link is in the body of your post.
If you upload images from your computer or similar in order to attach it to your post, that image will then be hosted on our Amazon server, meaning that, when the post goes out, if you click on the image or sometimes when it's displayed, particularly with Pinterest, it will display where the image is hosted which is why this is displayed.
To avoid this, the image would need to be hosted on another site and you would then need to attach it using the URL options. You can host images using a site like Photobucket or similar.