One of the reasons email clients often don't load images by default is because when an image is loaded the server it's pulled from can track that it was loaded,

One of the reasons email clients often don't load images by default is because when an image is loaded the server it's pulled from can track that it was loaded, and thus infer that the email has been opened. By loading images automatically, this tracking will again be possible. However, Google may be getting around this by loading all images on receipt by the Gmail servers and then caching them as the blog mentions. That way a sender will only be able to track that the email made it to Google, not whether the end user actually viewed it or not.