Gmail has changed the game by starting to cache the images. This will effect many things as you may be aware from the recient changes at your ESP reports and GA account.
Here are the list of things you should check and start thinking off;
The Google cache hides the source Internet Protocol (IP) address of the reader. That address can be used to determine the location of the reader. Gmail users now all appear to reside in Google datacenters.
Often, this is used for nothing more than drawing pretty maps showing readership around the world. But some companies also use it to provide more targeted information, like an image with the address of the outlet that is closest to you right now.
The referrer indicates what requested the image be downloaded. This data is heavily used by reports that tell you which folder your email ended up in and how many recipients were using the Gmail web interface versus a native client or the mobile app.
The user-agent string tells a site a lot about the program that is downloading the image. Browser version, operating system and device are often deduced from this. The impact of not having this is two-fold.
Statistics on email client and device usage will be unavailable for Gmail users, but also any tool that customizes images for the audience will be unable to function properly. So delivering a mobile-optimized image for a mobile device versus a desktop will not be viable (responsive design notwithstanding).
Cookie synchronization for ad serving and retargeting may be undermined. Advertisers are increasingly performing cookie synchronization between ad data management platforms and their CRM databases. The benefit is more relevant ad targeting and more timely and relevant email. Gmail may become a black hole for this activity.
Tools that deliver images dynamically based on time (think time-limited offers) will be negatively impacted. Once the recipient sees the image, it will not be downloaded again and so will not be able to change.
Since images are cached they will, at most, be downloaded once per recipient. Total opens will fall. Tools that use multiple image downloads to infer read times and repeat reading will be negatively impacted.
One other impact of the Google image cache is that it restricts images to 10MB. Any image larger than that produces an error. While not an issue for most, this can be significant for animated images.
Here is a post from Google’s official blog;
Have you ever wondered why Gmail asks you before showing images in emails? We did this to protect you from unknown senders who might try to use images to compromise the security of your computer or mobile device.
But thanks to new improvements in how Gmail handles images, you’ll soon see all images displayed in your messages automatically across desktop, iOS and Android. Instead of serving images directly from their original external host servers, Gmail will now serve all images through Google’s own secure proxy servers.
So what does this mean for you? Simple: your messages are more safe and secure, your images are checked for known viruses or malware, and you’ll never have to press that pesky “display images below” link again. With this new change, your email will now be safer, faster and more beautiful than ever.
Of course, those who prefer to authorize image display on a per message basis can choose the option “Ask before displaying external images” under the General tab in Settings. That option will also be the default for users who previously selected “Ask before displaying external content”.
Similar to existing features like default https access, suspicious activity detection, and free two-step verification, image proxying is another way your email is protected. This new improvement will be rolling out on desktop starting today and to your Gmail mobile apps in early 2014.