If you are a Gmail user you have probably gotten the popup upon login that images are now enabled by default. This coincides with Gmail announcing a couple of weeks ago that they will be caching and serving the images from their CDN to speed up browsing email.
You also have a option to disable this in your settings to go back to the “Ask me before displaying” just like the old Gmail setup.
Personally, I feel this is a new spam signal that Gmail will use to adjust their already great spam detection system.
The biggest single problem that Gmail has with spam is mass mailers (spammers) is using 100% images that contain keywords that would be flagged as spam. I am guessing they are trying to OCR (optical character recognition) but that is a monster in itself.
Also these spammers get around easy detection systems by sending from a slew of different IP addresses, subject lines, alt tags for images, and other items.
By caching these images Gmail can now use that as a signal when enough people report the email as spam and either not display the image, flag all emails with that image as spam, or even replace the image with a “this is spam” or something.
Many people have written various theories on this but only time will tell how its going to be implemented.