Christmas vacations spent well, without worrying too much of the digital world, and spending a lot of time with the family. I hope you, my reader, had some quality time as well.
I wrote about my first impressions on Google's image caching few weeks ago, and had some spare time to test the situation.
My tests weren't very wide, or with large recipient group, but it was consistent, and showed that the image caching is done "on-the-fly" by Google. And this was a relief. We can still have reports that show "how many percentage of recipients opened the email at least once", and even more, it's now even more reliable number, since the default setting in Gmail now is to show the pictures in each and every email.
If your email marketing system is sending images personalized, ie logo.jpg becomes crypted-text-that-includes-email-recipient-and-sendout-id.jpg (and I assume every commercial one is doing), you can track each individual's "first open", but not consecutive opens. So you will miss "cumulative open" or "sum of all opens" from Google Gmail service (and for Businesses, I assume), but that will not hurt a marketeer much.
The key reports are still successfully sent %, which gives you general idea of how current your email register is, and how well your messages are being sent and received by email servers (ie. not getting blacklisted), open %, which gives you good feedback on how well your "From" and "Subject" was perceived as interesting and trustworthy, and click %, which tells you how well your content was driving traffic to your site. With sophisticated systems you will continue into more important business reports, pull % over control group, how much revenue was created, and return-on-investment figures, if your system is integrated with your e-commerce and brick & mortar sales data.