E-mail marketers will no longer be able to get any information from images—they will see a single request from Google, which will then be used to send the image out to all Gmail users. Unless you click on a link, marketers will have no idea the e-mail has been seen.
But that doesn't seem to be the case for all marketers. In a blog post, e-mail marketing service MailChimp, noted that they (and most e-mail marketers) track opens by placing "placing a tiny, single-pixel-sized image in each email," so the system would allow them track the first open, but perhaps not repeat opens by subscriber.
In fact, MailChimp says, Google's explanation for how they are implementing their image caching system would actually make them "more accurate" when tracking unique open rates, because under the previous system the subscribers who opened emails without displaying images were effectively invisible to marketers. That's because previously, Gmail wouldn't display an image until users clicked a "show images" button. Because its image-caching system reduces the security and privacy risks from third-party images, Google is changing Gmail so that images load automatically when users open an email.
Google's own documentation seems to back up MailChimp's interpretation, with a support page stating that "senders may be able to know whether an individual has opened a message with unique image links." Similarly, Google confirmed to the Post that e-mail marketers using images to track open rates might see more representative open rates than they were before because they won't need users to both open and load images.
But Ars Technica is right that the Gmail change will limit the kind of information marketers get about e-mail recipients. For instance, marketers should no longer get users' IP addresses -- a valuable source of geo-location information from their perspective. And some of Google's previous moves have been pretty hard on e-mail marketers, with whom they compete for online advertising dollars. Many retail and non-profit e-mail list senders were upset to see a drop in open rates after being relegated to the "promotions" section when Gmail moved to a tabbed inbox structure over the summer.