Fun fact: Because I have images turned off by default in #Gmail and read the rest (more than half) of my email using a non-HTML MUA fed from my own SMTP servers, the senders of the vast majority of email I receive cannot know whether I've ever opened their emails or not. I think this is grand.
@lauren
I believe that Gmail replaces any remote image by a Google hosted version, so having images turned of there shouldn't make a difference
@edzilla You are misunderstanding what Gmail is doing. Yes, they will cache the first open of an image that is referenced in a received email. That means that subsequent opens of the same image won't track. BUT!!! Mail senders figured out how to get around this long ago, by sending unique tracking images to each recipient. This means that so long as the Gmail user has images enabled for a received message, the sender will get the open indication the first time the user reads the message. They won't get additional notifications on subsequent opens by that user, but that's much less important to them. So, the only real protection is to keep images OFF and only enable them on a message by message basis as necessary.