There's a lot of confusion around what they're really doing. Either: 1) they're caching the images when they're sent. This seems to be what Google and Ars are

There's a lot of confusion around what they're really doing. Either: 1) they're caching the images when they're sent. This seems to be what Google and Ars are saying, and it's what makes sense. 2) they're loading the image when the email is opened. This would be a privacy disaster, and is what Wired is saying, and from various comments people claim to have confirmed this by testing with their own servers.