[jpalomaki]

"When you get a message that has a picture in it, you see the picture automatically. "

https://support.google.com/mail/answer/145919?co=GENIE.Platf...

[vurpo]

Gmail automatically downloads and caches images, and loads them from their server instead of from the original remote server, when you read the email.

[timdorr]

That only obfuscates the reader's IP. Gmail respects Cache-Control headers, so it will not cache the image if you tell it not to.