When the email is sent, there is no record of the message. There is only an image URI that points to the image that contains the message. So, the NSA won't be able to grab a copy as it's sent. However, if an organization has the ability to intercept all images that are loaded in the browser (through the ISPs, for instance), it can grab a copy of the image containing the message. In that case, some OCR would have to be done in order to extract the message (which, agreed, is not hard to do if you are explicitly looking for messages contained within images).